FIX llvm-gcc bootstrap on x86_64 linux. If a virtual register is copied to a physical...

FIX llvm-gcc bootstrap on x86_64 linux. If a virtual register is copied to a physical register, it's not necessarily defined by a copy. We have to watch out it doesn't clobber any sub-register that might be live during its live interval. If the live interval crosses a basic block, then it's not safe to check with the less conservative check (by scanning uses and defs) because it's possible a sub-register might be live out of the block.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62144 91177308-0d34-0410-b5e6-96231b3b80d8