[llvm-mca] Move field 'AllowZeroMoveEliminationOnly' to class RegisterFile. NFC.

Flag 'AllowZeroMoveEliminationOnly' should have been a property of the PRF, and
not set at register granularity.

This change also restricts move elimination to writes that update a full
physical register. We assume that there is a strong correlation between
logical registers that allow move elimination, and how those same registers are
allocated to physical registers by the register renamer.

This is still a no functional change, because this experimental code path is
disabled for now. This is done in preparation for another patch that will add
the ability to describe how move elimination works in scheduling models.

llvm-svn: 343787