InstCombine: Don't miscompile X % ((Pow2 << A) >>u B)

We assumed that A must be greater than B because the right hand side of
a remainder operator must be nonzero.

However, it is possible for A to be less than B if Pow2 is a power of
two greater than 1.

Take for example:
i32 %A = 0
i32 %B = 31
i32 Pow2 = 2147483648

((Pow2 << 0) >>u 31) is non-zero but A is less than B.

This fixes PR21274.

llvm-svn: 219713