[CodeGenPrep] Handle constants in ConvertPhiType

This is a simple addition to the convertPhiTypes in CodeGenPrepare to
consider and convert constants as it converts the phi type. Someone
fixed the bug in the motivating example, so the undef is now a constant
0. This does mean converting between integer and floating point
constants, which may have different materialization.

Differential Revision: https://reviews.llvm.org/D135561