LoongArch: Use standard pattern name for xvfrsqrt/vfrsqrt instructions.

LoongArch: Use standard pattern name for xvfrsqrt/vfrsqrt instructions.

Rename lasx_xvfrsqrt*/lsx_vfrsqrt* to rsqrt<mode>2 to align with standard
pattern name. Define function use_rsqrt_p to decide when to use rsqrt optab.

gcc/ChangeLog:

	* config/loongarch/lasx.md (lasx_xvfrsqrt_<flasxfmt>): Renamed to ..
	(rsqrt<mode>2): .. this.
	* config/loongarch/loongarch-builtins.cc
	(CODE_FOR_lsx_vfrsqrt_d): Redefine to standard pattern name.
	(CODE_FOR_lsx_vfrsqrt_s): Ditto.
	(CODE_FOR_lasx_xvfrsqrt_d): Ditto.
	(CODE_FOR_lasx_xvfrsqrt_s): Ditto.
	* config/loongarch/loongarch.cc (use_rsqrt_p): New function.
	(loongarch_optab_supported_p): Ditto.
	(TARGET_OPTAB_SUPPORTED_P): New hook.
	* config/loongarch/loongarch.md (*rsqrt<mode>a): Remove.
	(*rsqrt<mode>2): New insn pattern.
	(*rsqrt<mode>b): Remove.
	* config/loongarch/lsx.md (lsx_vfrsqrt_<flsxfmt>): Renamed to ..
	(rsqrt<mode>2): .. this.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vector/lasx/lasx-rsqrt.c: New test.
	* gcc.target/loongarch/vector/lsx/lsx-rsqrt.c: New test.