When tail-merging multiple blocks, make sure to correctly update the live-in list...

When tail-merging multiple blocks, make sure to correctly update the live-in list on the merged block to correctly account for the live-outs of all the predecessors.  They might not be the same in all cases (the testcase I have involves a PHI node where one of the operands is an IMPLICIT_DEF).

Unfortunately, the testcase I have is large and confidential, so I don't have a test to commit at the moment; I'll see if I can come up with something smaller where this issue reproduces.

<rdar://problem/9716278>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134565 91177308-0d34-0410-b5e6-96231b3b80d8