Fix evil VirtRegRewriter bug.

Fix evil VirtRegRewriter bug.

The rewriter can keep track of multiple stack slots in the same register if they
happen to have the same value. When an instruction modifies a stack slot by
defining a register that is mapped to a stack slot, other stack slots in that
register are no longer valid.

This is a very rare problem, and I don't have a simple test case. I get the
impression that VirtRegRewriter knows it is about to be deleted, inventing a
last opaque problem.

<rdar://problem/9204040>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128562 91177308-0d34-0410-b5e6-96231b3b80d8