mips: check MSA support for vector modes [PR100760,PR100761,PR100762]

mips: check MSA support for vector modes [PR100760,PR100761,PR100762]

Check if the vector mode is really supported by MSA in certain cases,
instead of testing ISA_HAS_MSA.  Simply testing ISA_HAS_MSA can cause
ICE when MSA is enabled besides other MIPS SIMD extensions (notably,
Loongson MMI).

gcc/

	PR target/100760
	PR target/100761
	PR target/100762
	* config/mips/mips.c (mips_const_insns): Use MSA_SUPPORTED_MODE_P
	instead of ISA_HAS_MSA.
	(mips_expand_vec_unpack): Likewise.
	(mips_expand_vector_init): Likewise.

gcc/testsuite/

	PR target/100760
	PR target/100761
	PR target/100762
	* gcc.target/mips/pr100760.c: New test.
	* gcc.target/mips/pr100761.c: New test.
	* gcc.target/mips/pr100762.c: New test.