[InstCombine] Fold or(zext(bswap(x)),shl(zext(bswap(y)),bw/2)) ->...

[InstCombine] Fold or(zext(bswap(x)),shl(zext(bswap(y)),bw/2)) -> bswap(or(zext(x),shl(zext(y), bw/2))

This adds a general combine that can be used to fold:

  or(zext(OP(x)), shl(zext(OP(y)),bw/2))
-->
  OP(or(zext(x), shl(zext(y),bw/2)))

Allowing us to widen 'concat-able' style or+zext patterns - I've just set this up for BSWAP but we could use this for other similar ops (BITREVERSE for instance).

We already do something similar for bitop(bswap(x),bswap(y)) --> bswap(bitop(x,y))

Fixes PR45715

Reviewed By: @lebedev.ri

Differential Revision: https://reviews.llvm.org/D79041