Clear kill flags on the fly when joining intervals.

Clear kill flags on the fly when joining intervals.

With physreg joining out of the way, it is easy to recognize the
instructions that need their kill flags cleared while testing for
interference.

This allows us to skip the final scan of all instructions for an 11%
speedup of the coalescer pass.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157169 91177308-0d34-0410-b5e6-96231b3b80d8