[x86] shrink 'and' immediate values by setting the high bits (PR35907)

  
Try to reverse the constant-shrinking that happens in SimplifyDemandedBits()
for 'and' masks when it results in a smaller sign-extended immediate.

We are also able to detect dead 'and' ops here (the mask is all ones). In
that case, we replace and return without selecting the 'and'.

Other targets might want to share some of this logic by enabling this under a
target hook, but I didn't see diffs for simple cases with PowerPC or AArch64,
so they may already have some specialized logic for this kind of thing or have
different needs.

This should solve PR35907:
https://bugs.llvm.org/show_bug.cgi?id=35907

Differential Revision: https://reviews.llvm.org/D42088

llvm-svn: 322957

0 files changed, 0 insertions, 0 deletions