AMDGPU: Improve accuracy of instruction rates for VOPC

AMDGPU: Improve accuracy of instruction rates for VOPC

These were all using the default 32-bit VALU write class,
but the i64/f64 compares are half rate.

I'm not sure this is really correct, because they are still using
the write to VALU write class, even though they really write
to the SALU.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248582 91177308-0d34-0410-b5e6-96231b3b80d8