[InstCombine] Simplify bswap -> shift

Simplify bswap(x) to shl(x) or lshr(x) if x has exactly one
"active byte", i.e. all active bits are contained in boundaries
of a single byte of x.

https://alive2.llvm.org/ce/z/nvbbU5
https://alive2.llvm.org/ce/z/KiiL3J

Reviewed By: spatel, craig.topper, lebedev.ri

Differential Revision: https://reviews.llvm.org/D117680