AMDGPU/GlobalISel: Make IMPLICIT_DEF of all sizes < 512 legal.

Summary:
We could split sizes that are not power of two into smaller sized
G_IMPLICIT_DEF instructions, but this ends up generating
G_MERGE_VALUES instructions which we then have to handle in the instruction
selector.  Since G_IMPLICIT_DEF is really a no-op it's easier just to
keep everything that can fit into a register legal.

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, rovka, kristof.beyls, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D48777

llvm-svn: 336041