Recompute kill flags from live intervals after coalescing instead of trying to

Recompute kill flags from live intervals after coalescing instead of trying to
update them. Computing kill flags is notoriously difficult, and the coalescer
would get it wrong sometimes, and it would completely skip physical registers.

Now we simply remove kill flags based on the live intervals after coalescing.
This is a few percent slower, but now we get correct kill flags for physical
registers after coalescing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102510 91177308-0d34-0410-b5e6-96231b3b80d8