x86jit: Allow only one reg for sequential load.
x86jit: Find the correct available regs.

If there aren't n available regs, they may not of course be in slots 0 and
1.  Have to go and find them.
x86jit: when reusing a reg, don't zap slot.

If we returned the same reg it was in before, don't zap the slots because
slot #1 may be valid.
Merge pull request #7118 from unknownbrackets/jit-simd2

x86jit: Correctly use available regs for simd load