MIPS: Add MSUBF.fmt instruction for MIPSr6

MIPS: Add MSUBF.fmt instruction for MIPSr6

GCC currently uses two instructions (NEG.fmt and MADDF.fmt) for
operations like `x - (y * z)' for MIPSr6. We can further tune this by
using only MSUBF.fmt instead of those two.

This patch adds MSUBF.fmt instrutions with corresponding tests.

gcc/ChangeLog:

	* config/mips/mips.md (fms<mode>4): Generates MSUBF.fmt
	instructions.
	(*fms<mode>4_msubf): Same as above.
	(fnma<mode>4): Same as above.
	(*fnma<mode>4_msubf): Same as above.

gcc/testsuite/ChangeLog:

	* gcc.target/mips/mips-msubf.c: New tests for MIPSr6.