[MIR-Canon] Add support for rewriting VRegs that are typed but don't have an RC.

There were crashes (addrspace-memoperands.mir was only one of them) in MIR that
had operands that came from before register classes were set. With these
operands, creating a replacement vreg (for MIR-Canon's renaming) needs to use
the vreg type rather than the RegisterClass which is not present.

Differential Revision: https://reviews.llvm.org/D62543

llvm-svn: 362122