[llvm] [MIPS]Remove unnecessary SLL instructions on MIPS64el (PR #109386)

[Craig Topper via llvm-commits]

topperc wrote:

> I don't think this transform is correct: https://alive2.llvm.org/ce/z/-kNt7y

I agree. You either need to call ComputeNumSignBits on the truncate input or do this transform instead:

((signext (xor (trunc X), imm)) to (xor (signext_inreg X, i32), imm).

https://github.com/llvm/llvm-project/pull/109386