[MCA][RegisterFile] Consistently update the PRF in the presence of multiple...

[MCA][RegisterFile] Consistently update the PRF in the presence of multiple writes to the same register.

My last change to the RegisterFile (PR51495) has introduced a bug in the logic
that allocates physical registers in the PRF.

In some cases, this bug could have triggered a nasty unsigned wrap in the number
of allocated registers, thus resulting in mca being stuck forever in a loop of
PRF availability checks.