[InstCombine] Recommit: Shift amount reassociation: shl-trunc-shl pattern

This was initially committed in r368059 but got reverted in r368084
because there was a faulty logic in how the shift amounts type mismatch
was being handled (it simply wasn't).

I've added an explicit bailout before we SimplifyAddInst() - i don't think
it's designed in general to handle differently-typed values, even though
the actual problem only comes from ConstantExpr's.

I have also changed the common type deduction, to not just blindly
look past zext, but try to do that so that in the end types match.

Differential Revision: https://reviews.llvm.org/D65380

llvm-svn: 368141