[TargetPassConfig] Run MachineVerifier after more passes.

We were disabling verification for no reason in a bunch of places; just
turn it on.

At this point, there are two key places where we don't run verification:
during register allocation, and after addPreEmitPass.  Regalloc probably
isn't worth messing with; it has its own invariants, and verifying
afterwards is probably good enough.  For after addPreEmitPass, it's
probably worth investigating improvements.