RegisterCoalescer: Forcibly leave SSA to avoid MIR test errors

Not sure how to produce a test that demonstrates the problem
today. The coalescer would have to introduce a verifier caught SSA
violation, like multiple defs of a virtual register. I'm not sure what
would do that now, but an upcoming patch will.

https://reviews.llvm.org/D156271

GitOrigin-RevId: 32a23aecf8002e181eb1022b8733ef8666b3241f