Extend switch condition in optimizeSwitchPhiConst when free

In a case like:

    switch((i32)x) { case 42: phi((i64)42, ...); }

replace `(i64)42` with `zext(x)` when we can do so for free.

This fixes a part of https://github.com/llvm/llvm-project/issues/55153

Differential Revision: https://reviews.llvm.org/D124897