Yingwei Zheng 47fd32f81c [DAGCombine] Fix type mismatch in (shl X, cttz(Y)) -> (mul (Y & -Y), X) (#94008) ...

Proof: https://alive2.llvm.org/ce/z/J7GBMU

Same as https://github.com/llvm/llvm-project/pull/92753, the types of
LHS and RHS in shift nodes may differ.
+ When VT is smaller than ShiftVT, it is safe to use trunc.
+ When VT is larger than ShiftVT, it is safe to use zext iff
`is_zero_poison` is true (i.e., `opcode == ISD::CTTZ_ZERO_UNDEF`). See
also the counterexample `src_shl_cttz2 -> tgt_shl_cttz2` in the alive2
proofs.

Fixes issue
https://github.com/llvm/llvm-project/pull/85066#issuecomment-2142553617.

2024-06-01 19:04:55 +08:00

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