[InstSimplify] add folds for constant mask of value shifted by constant

We would eventually catch these via demanded bits and computing known bits in InstCombine,
but I think it's better to handle the simple cases as soon as possible as a matter of efficiency.

This fold allows further simplifications based on distributed ops transforms. eg:
  %a = lshr i8 %x, 7
  %b = or i8 %a, 2
  %c = and i8 %b, 1

InstSimplify can directly fold this now:
  %a = lshr i8 %x, 7

Differential Revision: https://reviews.llvm.org/D33221

llvm-svn: 303213