InstCombine: Don't miscompile (x lshr C1) udiv C2

We have a transform that changes:
  (x lshr C1) udiv C2
into:
  x udiv (C2 << C1)

However, it is unsafe to do so if C2 << C1 discards any of C2's bits.

This fixes PR21255.

llvm-svn: 219634