[InstCombine] Optimize select(freeze(icmp eq/ne x, y), x, y)
This patch adds an optimization that folds select(freeze(icmp eq/ne x, y), x, y)
to x or y.
This was needed to resolve slowdown after D84940 is applied.
I tried to bake this logic into foldSelectInstWithICmp, but it wasn't clear.
This patch conservatively writes the pattern in a separate function,
foldSelectWithFrozenICmp.
The output does not need freeze; https://alive2.llvm.org/ce/z/X49hNE (from @nikic)
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D85533