[InstCombine] recognizeBSwapOrBitReverseIdiom - support for 'partial' bswap patterns (PR47191)

If we're bswap'ing some bytes and zero'ing the remainder we can perform this as a bswap+mask which helps us match 'partial' bswaps as a first step towards folding into a more complex bswap pattern.

Differential Revision: https://reviews.llvm.org/D88578