[InstCombine] Make MatchBSwap also match bit reversals

[InstCombine] Make MatchBSwap also match bit reversals

MatchBSwap has most of the functionality to match bit reversals already. If we switch it from looking at bytes to individual bits and remove a few early exits, we can extend the main recursive function to match any sequence of ORs, ANDs and shifts that assemble a value from different parts of another, base value. Once we have this bit->bit mapping, we can very simply detect if it is appropriate for a bswap or bitreverse.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255334 91177308-0d34-0410-b5e6-96231b3b80d8