LoongArch: Standard instruction template fnmam4 correction

LoongArch: Standard instruction template fnmam4 correction

The current implementation of the fnmam4 instruction template requires
the third source operand to be assigned the same hard register as the
target operand, but the constraint is not documented in the instruction
manual or standard template definitions. The current constraint will
generate additional data dependencies and extra instructions.

gcc/ChangeLog:

	* config/loongarch/lasx.md (fnma<mode>4): Remove.
	* config/loongarch/lsx.md (fnma<mode>4): Remove.
	* config/loongarch/simd.md (fnma<mode>4): Simplify and correct.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/fnmam4-vec.c: New test.