MMX/x87 interaction is subtly broken

[alyssarosenzweig]

MMX is supposed to set the upper bits of the 80-bit x87 register to -1, and also supposed to set certain x87 flags. FEX isn't doing either. I don't know of any applications that rely on this behaviour, but Intel documents it explicitly.

[alyssarosenzweig]

see dca0906 for what prompted this. FEX is currently implicitly inserting without touching the upper bits, which is wrong. With that commit, FEX explicitly inserts and explicitly preserves the upper bits, which is slow and just as wrong.

[pmatos]

Due to a couple of other blockers, I might look into this earlier than I thought I would.