[AMDGPU] Fix illegal agpr use by VALU

When SIFixSGPRCopies attempts to fix an illegal copy from vector to
scalar register it calls moveToVALU(). A copy from an agpr to sgpr
becomes a copy from agpr to agpr, which may result in the illegal
register class at a use of this copy.

Solution is to copy it always into a vgpr. This may result in a
subsequent copy into an agpr if that is what really needed, however
should not happen too often and likely will be folded later.

The opposite situation may not happen because an sgpr is always
illegal where agpr is legal, so such user instructions may not
exist.

Differential Revision: https://reviews.llvm.org/D68358

llvm-svn: 373544