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

[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)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182053 91177308-0d34-0410-b5e6-96231b3b80d8