[LegalizeTypes] In SoftenFloatRes_FP_EXTEND, move the check for input already being...

[LegalizeTypes] In SoftenFloatRes_FP_EXTEND, move the check for input already being promoted above the check for fp16 converting to something other than fp32.

The fp16 to larger than fp32 inserts an extend that need to
re-legalized if fp16 is promoted. But if we check for fp16
promotion first, then we can avoid emiting the fp_extend all
together.