R600/SI: Don't verify constant bus usage of flag ops

R600/SI: Don't verify constant bus usage of flag ops

This was checking if pseudo-operands like the source
modifiers were using the constant bus, which happens to work
because the values these all can be happen to be valid inline
immediates.

This fixes a later commit which starts checking the register class
of the operands.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224078 91177308-0d34-0410-b5e6-96231b3b80d8