[InstCombine] Bubble vector.reverse of select operands to their result.

This mirrors a similar shufflevector transformation so the same
effect is obtained for scalable vectors. The transformation is
only performed when it can be proven the number of resulting
reversals is not increased. By bubbling the reversals from operand
to result this should typically be the case and ideally leads to
back-back shuffles that can be elimitated entirely.

Differential Revision: https://reviews.llvm.org/D139339