[GlobalISel] Tweak lowering of G_SMULO/G_UMULO

Summary:
Applying this cleanup:

    -      MIRBuilder.buildInstr(TargetOpcode::G_ASHR)
    -        .addDef(Shifted)
    -        .addUse(Res)
    -        .addUse(ShiftAmt);
    +      MIRBuilder.buildAShr(Shifted, Res, ShiftAmt);

caused an assertion failure here:

    llc: /home/jayfoad2/git/llvm-project/llvm/lib/CodeGen/MachineRegisterInfo.cpp:404: llvm::MachineInstr *llvm::MachineRegisterInfo::getVRegDef(unsigned int) const: Assertion `(I.atEnd() || std::next(I) == def_instr_end()) && "getVRegDef assumes a single definition or no definition"' failed.

    #4  0x00000000050a6d96 in llvm::MachineRegisterInfo::getVRegDef (this=0x74606a0, Reg=2147483650) at /home/jayfoad2/git/llvm-project/llvm/lib/CodeGen/MachineRegisterInfo.cpp:403
    #5  0x00000000066148f6 in llvm::getConstantVRegValWithLookThrough (VReg=2147483650, MRI=..., LookThroughInstrs=false, HandleFConstant=true) at /home/jayfoad2/git/llvm-project/llvm/lib/CodeGen/GlobalISel/Utils.cpp:244
    #6  0x00000000066147da in llvm::getConstantVRegVal (VReg=2147483650, MRI=...) at /home/jayfoad2/git/llvm-project/llvm/lib/CodeGen/GlobalISel/Utils.cpp:210
    #7  0x0000000006615367 in llvm::ConstantFoldBinOp (Opcode=101, Op1=2147483650, Op2=2147483656, MRI=...) at /home/jayfoad2/git/llvm-project/llvm/lib/CodeGen/GlobalISel/Utils.cpp:341
    #8  0x000000000657eee0 in llvm::CSEMIRBuilder::buildInstr (this=0x7465010, Opc=101, DstOps=..., SrcOps=..., Flag=...) at /home/jayfoad2/git/llvm-project/llvm/lib/CodeGen/GlobalISel/CSEMIRBuilder.cpp:160
    #9  0x0000000003645958 in llvm::MachineIRBuilder::buildAShr (this=0x7465010, Dst=..., Src0=..., Src1=..., Flags=...) at /home/jayfoad2/git/llvm-project/llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h:1298
    #10 0x00000000065c35b1 in llvm::LegalizerHelper::lower (this=0x7fffffffb5f8, MI=..., TypeIdx=0, Ty=...) at /home/jayfoad2/git/llvm-project/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp:2020

because at this point there are two instructions defining Res: the
original G_SMULO/G_UMULO and the new G_MUL that we built. The fix is
to modify the original mul in place, so that there is only ever one
definition of Res.

Reviewers: arsenm, aditya_nandakumar

Subscribers: wdng, rovka, hiraditya, volkan, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72842

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -2131,36 +2131,32 @@ LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT Ty) {
     Register LHS = MI.getOperand(2).getReg();
     Register RHS = MI.getOperand(3).getReg();
 
-    MIRBuilder.buildMul(Res, LHS, RHS);
-
     unsigned Opcode = MI.getOpcode() == TargetOpcode::G_SMULO
                           ? TargetOpcode::G_SMULH
                           : TargetOpcode::G_UMULH;
 
-    Register HiPart = MRI.createGenericVirtualRegister(Ty);
-    MIRBuilder.buildInstr(Opcode)
-      .addDef(HiPart)
-      .addUse(LHS)
-      .addUse(RHS);
+    Observer.changingInstr(MI);
+    const auto &TII = MIRBuilder.getTII();
+    MI.setDesc(TII.get(TargetOpcode::G_MUL));
+    MI.RemoveOperand(1);
+    Observer.changedInstr(MI);
+
+    MIRBuilder.setInsertPt(MIRBuilder.getMBB(), ++MIRBuilder.getInsertPt());
+
+    auto HiPart = MIRBuilder.buildInstr(Opcode, {Ty}, {LHS, RHS});
 
     Register Zero = MRI.createGenericVirtualRegister(Ty);
     MIRBuilder.buildConstant(Zero, 0);
 
     // For *signed* multiply, overflow is detected by checking:
     // (hi != (lo >> bitwidth-1))
     if (Opcode == TargetOpcode::G_SMULH) {
-      Register Shifted = MRI.createGenericVirtualRegister(Ty);
-      Register ShiftAmt = MRI.createGenericVirtualRegister(Ty);
-      MIRBuilder.buildConstant(ShiftAmt, Ty.getSizeInBits() - 1);
-      MIRBuilder.buildInstr(TargetOpcode::G_ASHR)
-        .addDef(Shifted)
-        .addUse(Res)
-        .addUse(ShiftAmt);
+      auto ShiftAmt = MIRBuilder.buildConstant(Ty, Ty.getSizeInBits() - 1);
+      auto Shifted = MIRBuilder.buildAShr(Ty, Res, ShiftAmt);
       MIRBuilder.buildICmp(CmpInst::ICMP_NE, Overflow, HiPart, Shifted);
     } else {
       MIRBuilder.buildICmp(CmpInst::ICMP_NE, Overflow, HiPart, Zero);
     }
-    MI.eraseFromParent();
     return Legalized;
   }
   case TargetOpcode::G_FNEG: {