R600/SI: Stop using VSrc_* as the default register class for types.

We now use SReg_* for integer types and VReg_* for floating-point types.
This should help simplify the SIFixSGPRCopies pass and no longer causes
ISel to insert a COPY after termiator instuctions that output a value.

This change is covered by exisitng tests.

llvm-svn: 208888