Commits on Jan 19, 2024

1. Fix phase order problem with throw helper blocks …

   Throw helper blocks are created in morph, then possibly removed if
   unnecessary in StackLevelSetter (under optimization). However, there
   was a case where StackLevelSetter removed an OVERFLOW throw helper
   block after optimization proved it unnecessary because of a constant
   zero dividend, but between StackLevelSetter and codegen, LSRA introduced
   a RELOAD node above the constant zero that `GenTree::CanDivOrModPossiblyOverflow()`
   didn't understand, thus causing it to think that overflow was possible.
   Codegen looked for the OVERFLOW throw helper block and couldn't find it.
   
   There are multiple fixes here, somewhat "defense in depth":
   - If `StackLevelSetter::SetThrowHelperBlocks()` determines a node can't throw
   divide-by-zero or ArithmeticException (overflow), it marks the node
   GTF_DIV_MOD_NO_BY_ZERO / GTF_DIV_MOD_NO_OVERFLOW, respectively. This is what
   morph does earlier in compilation.
   - `genMarkLabelsForCodegen()` does not mark throw helper blocks where `acdUsed`
   is false, to avoid marking deleted blocks.
   - More asserts are added that `acdUsed` is true when codegen goes to generate
   a branch to a throw helper.
   - `GenTree::OperExceptions` / `CanDivOrModPossiblyOverflow` are changed to skip
   COPY/RELOAD nodes.
   
   Fixes dotnet#96224

   

   BruceForstall committed Jan 19, 2024
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