Fix FXTRACT for 0.0 and -0.0

[pmatos]

Fixes fxtract by returning the correct values for 0.0 and -0.0. We moved the split of fxtract into _sig and _exp, to the opcode dispatcher, to ease some comparisons.

Also removed the IR node F80XTRACTStack which is not needed anymore.

Depends on landing #4092 (commit d62dd50 included here but to be merged separately)

[pmatos]

For context the spec mentions that for fscale:

The FSCALE instruction can also be used to reverse the action of the FXTRACT instruction, as shown in the following example:

FXTRACT;
FSCALE;
FSTP ST(1);

When testing this, through FScaleFXtract.asm unit test, I noticed that for 0.0 and -0.0, fxtract was generating incorrect values. I fixed these there.

[pmatos]

All good here now - instcounci failure coming from #4092 that needs to land before this one.

[pmatos]

pinging here for review!
One thing I would like another pair of eyes on is the IR definition of NZCVSelectV. Do we actually need to specify a ResultSize? Can't we just say that the ResultSize is the same size as the TrueVal (or FalseVal). Also maybe we need to validate that these have the same size?

[Sonicadvance1]

Seems reasonable to me, but I'll leave it to Alyssa for the IR change.

[neobrain]

Hardcoded constants with lots of zeroes like these are prone to typos that aren't easily caught in code review either. There's two ways to avoid this issue:

• Compute the constant symbolically: 1UL << 63
• Use digit separators to make double-checking easier: 0x8000'0000'0000'0000UL

(Mentioning this here because the PR adds a bunch such constants and I stopped counting zeroes after the first one)