Causes Portal to go from 85FPS to 120FPS on Lenovo X13s.
When running in 32-bit mode we were wasting 8 GPRs and 8 FPRs by still
allocating the top 8 registers of each even though 32-bit can't use
them.
Reallocate them to be register allocated registers when running 32-bit
applications which reduces spills and lowers the cost of
spilling/filling SRA registers.
Quite a significant speed boost for a little bit of work.
Ryan Houdek
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