[InstCombine] recognizeBSwapOrBitReverseIdiom - recognise zext(bswap(trunc(x))) patterns (PR39793)

PR39793 demonstrated an issue where we fail to recognize 'partial' bswap patterns of the lower bytes of an integer source.

In fact, most of this is already in place collectBitParts suitably tags zero bits, so we just need to correctly handle this case by finding the zero'd upper bits and reducing the bswap pattern just to the active demanded bits.

Differential Revision: https://reviews.llvm.org/D88316