[PowerPC][Builtin] Allowing __rlwnm to accept a variable as a shift parameter

The builtin __rlwnm is currently constrained to accept only constants
for the shift parameter but the instructions emitted for it have no such
constraint, this patch allows the builtins to accept variable shift.

Reviewed By: NeHuang, amyk

Differential Revision: https://reviews.llvm.org/D111229