[mips] Improve instruction selection for pattern (store (fp_to_sint $src), $ptr).

Previously, three instructions were needed:

trunc.w.s $f0, $f2
mfc1 $4, $f0
sw $4, 0($2)

Now we need only two:

trunc.w.s $f0, $f2
swc1 $f0, 0($2)

llvm-svn: 182053