Change materializeFrameBaseRegister() to return register

The only caller of this function is in the LocalStackSlotAllocation
and it creates base register of class returned by the target's
getPointerRegClass(). AMDGPU wants to use a different reg class
here so let materializeFrameBaseRegister to just create and return
whatever it wants.

Differential Revision: https://reviews.llvm.org/D95268