[SelectionDAG] Optimize bitreverse expansion to minimize the number of mask constants.

We can halve the number of mask constants by masking before shl
and after srl.

This can reduce the number of mov immediate or constant
materializations. Or reduce the number of constant pool loads
for X86 vectors.

I think we might be able to do something similar for bswap. I'll
look at it next.

Differential Revision: https://reviews.llvm.org/D108738