our current machine instruction defines a register with the same register class
as what's being replaced. This showed up in the SPEC 403.gcc benchmark, where it
would ICE because a tail call was expecting one register class but was given
another. (The machine instruction verifier catches this situation.)
<rdar://problem/10270968>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141830 91177308-0d34-0410-b5e6-96231b3b80d8
