revert r301766: InstructionSimplify: Canonicalize shuffle operands. NFC-ish

Turns out this wasn't NFC-ish at all because there's a bug processing shuffles
that change the size of their input vectors (that case always seems to trip us
up). 

This should fix PR32872 while we investigate how it failed and reduce a testcase:
https://bugs.llvm.org/show_bug.cgi?id=32872
 

llvm-svn: 301977