[DAG] SelectionDAG::canCreateUndefOrPoison - add ISD::AVGFLOORS/AVGFLOORU/AVGCEILS/AVGCEILU handling + tests

[RKSimon]

• Create alive2 tests proving AVGFLOOR / AVGCEIL patterns don't create poison/undef
• Add ISD::AVGFLOORS/AVGFLOORU/AVGCEILS/AVGCEILU to SelectionDAG::canCreateUndefOrPoison and add suitable test coverage

[llvmbot]

Hi!

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

- [ ] Create alive2 tests proving AVGFLOOR / AVGCEIL patterns don't create poison/undef - [ ] Add ISD::AVGFLOORS/AVGFLOORU/AVGCEILS/AVGCEILU to SelectionDAG::canCreateUndefOrPoison and add suitable test coverage

[aabhinavg1]

Hi @RKSimon
#148191 marks ISD::AVGFLOORS and ISD::AVGCEILS as safe in SelectionDAG::canCreateUndefOrPoison.

Verified using Alive2 that these operations do not introduce poison or undef:

• AVGFLOORS proof (Alive2)
• AVGCEILS proof (Alive2)

Let me know if further clarification or changes are needed. Thanks!