MachineSSAUpdater: Allow initialization with just a register class

The register class is required for inserting PHIs, but the "current
virtual register" isn't actually used for anything, so let's remove it
while we're at it.

Differential Revision: https://reviews.llvm.org/D85602

Change-Id: I1e647f31570ef21a7ea8e20db3454178e98a6a8b