GlobalISel: Use a callback to compute constrained reg class for unallocatble registers

Summary:
constrainOperandRegClass() currently fails if it tries to constrain the
register class of an operand that is defeined with an unallocatable register
class.  This patch resolves this by adding a target callback to compute
register constriants in this case.

This is required by the AMDGPU because many of its instructions have source opreands
defined with the unallocatable register classe VS_32 which is a union of two allocatable
register classes VGPR_32 and SReg_32.

Reviewers: dsanders, aditya_nandakumar

Reviewed By: aditya_nandakumar

Subscribers: rovka, kristof.beyls, tpr, llvm-commits

Differential Revision: https://reviews.llvm.org/D45991

llvm-svn: 331485