[RegAllocFast] Scan physcial reg definitions before assigning virtual reg definitions
When assigning the definitions of an instruction we were updating
the available registers while walking the definitions. Some of
those definitions may be from physical registers and thus, they are
not available for other definitions to take, but by the time we see
that we may have already assign these registers to another
Fix that by walking through all the definitions and mark as unavailable
the physical register definitions, then do the virtual register assignments.