[SelectionDAG] Optimization of BITREVERSE legalization for power-of-2 integer scalar/vector types

An extension of D19978, this patch replaces the default BITREVERSE evaluation of individual bit masks+shifts with block mask+shifts when we have integer elements of power-of-2 bits in size.

After calling BSWAP to reverse the order of the constituent bytes (which typically follows a similar approach), every neighbouring 4-bits, 2-bits and finally 1-bit pairs are masked off and swapped over with shifts.

In doing so we can significantly reduce the number of operations required.

Differential Revision: https://reviews.llvm.org/D21578

llvm-svn: 276432