R600/SI: Use VSrc_* register classes as the default classes for types

R600/SI: Use VSrc_* register classes as the default classes for types

Since the VSrc_* register classes contain both VGPRs and SGPRs, copies
that used be emitted by isel like this:

SGPR = COPY VGPR

Will now be emitted like this:

VSrC = COPY VGPR

This patch also adds a pass that tries to identify and fix situations where
a VGPR to SGPR copy may occur.  Hopefully, these changes will make it
impossible for the compiler to generate illegal VGPR to SGPR copies.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187831 91177308-0d34-0410-b5e6-96231b3b80d8