[InstCombine] Host inversion out of ashr's value operand (PR48995)

This is a yet another hint that we will eventually need InstCombineInverter,
which would consistently sink inversions, but but for that we'll need
to consistently hoist inversions where possible, so let's do that here.

Example of a proof: https://alive2.llvm.org/ce/z/78SbDq

See https://bugs.llvm.org/show_bug.cgi?id=48995