[InstCombine] try to narrow shifted bswap-of-zext

This is the IR counterpart to 370ebc9d
which provided a bswap narrowing fix for issue #53867.

Here we can be more general (although I'm not sure yet
what would happen for illegal types in codegen - too
rare to worry about?):
https://alive2.llvm.org/ce/z/3-CPfo

This will be more effective if we have moved the shift
after the bswap as proposed in D122010, but it is
independent of that patch.

Differential Revision: https://reviews.llvm.org/D122166