Reduce number of spill instructions generated by VEX register allocator v3.

Reduce number of spill instructions generated by VEX register allocator v3.

Keeps track whether the bound real register has been reloaded from a virtual
register recently and if this real reg is still equal to that spill slot.
Avoids unnecessary spilling that vreg later, when this rreg needs
to be reserved, usually as a caller save register for a helper call.

Fixes BZ#384526.