[InstCombine] fix shuffle-of-binops transform to avoid poison/undef

As noted in D48987, there are many different ways for this transform to go wrong. 
In particular, the poison potential for shifts means we have to more careful with those ops. 
I added tests to make that behavior visible for all of the different cases that I could find.

This is a partial fix. To make this review easier, I did not make changes for the single binop 
pattern (handled in foldSelectShuffleWith1Binop()). I also left out some potential optimizations 
noted with TODO comments. I'll follow-up once we're confident that things are correct here.

The goal is to correct all marked FIXME tests to either avoid the shuffle transform or do it safely.

Note that distinguishing when the shuffle mask contains undefs and using getBinOpIdentity() allows 
for some improvements to div/rem patterns, so there are wins along with the missed opportunities 
and fixes.

Differential Revision: https://reviews.llvm.org/D49047

llvm-svn: 336546