R600: Implement TargetInstrInfo::isLegalToSplitMBBAt()

R600: Implement TargetInstrInfo::isLegalToSplitMBBAt()

Splitting a basic block will create a new ALU clause, so we need to make
sure we aren't moving uses of registers that are local to their
current clause into a new one.

I had a test case for this, but unfortunately unrelated schedule changes
invalidated it, and I wasn't been able to come up with another one.

NOTE: This is a candidate for the 3.4 branch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195399 91177308-0d34-0410-b5e6-96231b3b80d8