[mips] Fix instruction selection pattern for sint_to_fp node to avoid emitting an
invalid instruction sequence.

Rather than emitting an int-to-FP move instruction and an int-to-FP conversion
instruction during instruction selection, we emit a pseudo instruction which gets
expanded post-RA. Without this change, register allocation can possibly insert a
floating point register move instruction between the two instructions, which is not
valid according to the ISA manual.

mtc1 $f4, $4         # int-to-fp move instruction.
mov.s $f2, $f4       # move contents of $f4 to $f2.
cvt.s.w $f0, $f2     # int-to-fp conversion.