CodeGenPrepare: Replace constant PHI arguments with switch condition value

We often see code like the following after running SCCP:

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

This tends to produce bad code as we currently materialize the constant
phi-argument in the switch-block. This increases register pressure and
if the pattern repeats for `n` case statements, we end up generating `n`
constant values.

This changes CodeGenPrepare to catch this pattern and revert it back to:

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

Differential Revision: https://reviews.llvm.org/D124552