Jit64: fix fmrx regression

Revision ddaf29e039d1736be1ca48c1c0b6af1c8d4ec47d introduced a register
corruption bug (#6825). Since fmrx/MOVSD only modifies ps0 but we save
both ps0 and ps1 in one xmm register, not loading the previous value
when binding to a x64 register trashed ps1.

But hey, a good opportunity to shave off one more instruction ;)
Revert "Handle BP mask register better to avoid useless BP writes (causing flushes)"

This reverts commit 954be9e2d9bfca8b33ac1cb36d3226bf2555f2c1.

Fixes issue 6826.
jit64: add regcache option IsBound

Lots of x86 instructions are different on memory vs registers.
So to generate code, we often have to check if a ppc register is already bound to a x86 register.
Jit64: really fix fmrx regression

This is more tricky than I thought!