JIT: Misc if-conversion

[BoyBaykiller]

• Allow GT_SELECT for floats, but only temporally for optimizations, not into codegen (for now)
• Call gtFoldExprConditional, this handles simple folding
• Add missing GT_SELECT equality, needed because without gtFoldExprConditional asserts
• Move the 32-bit bailout to after optimizations so that it can still profit from them. Only abort when there were no optimizations done

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[tannergooding]

Allow SELECTs for floats (only temporally for optimizations, not into codegen)

There's notably no reason to not allow it into codegen if the input is a floating-point comparison.

Such cases can emit cmpss followed by blendvps, which is smaller than ucomiss; test; jcc; etc

[BoyBaykiller]

That'd be nice to handle. Currently doing that hits assert isGeneralRegister(dstReg) && isGeneralRegister(srcReg) in emitxarch.cpp:7955 so that probably needs some extra work (different PR)

[BoyBaykiller]

@jakobbotsch PTAL. A few tiny changes packed in one PR.
Not quite sure how this is supposedly a slight tp improvement diffs.

Next we can look into supporting float-select into codegen. Or adding opts for it.