Support variable index vec_set.

Support variable index vec_set.

gcc/ChangeLog:

	PR target/97194
	* config/i386/i386-expand.c (ix86_expand_vector_set_var): New function.
	* config/i386/i386-protos.h (ix86_expand_vector_set_var): New Decl.
	* config/i386/predicates.md (vec_setm_operand): New predicate,
	true for const_int_operand or register_operand under TARGET_AVX2.
	* config/i386/sse.md (vec_set<mode>): Support both constant
	and variable index vec_set.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx2-vec-set-1.c: New test.
	* gcc.target/i386/avx2-vec-set-2.c: New test.
	* gcc.target/i386/avx512bw-vec-set-1.c: New test.
	* gcc.target/i386/avx512bw-vec-set-2.c: New test.
	* gcc.target/i386/avx512f-vec-set-2.c: New test.
	* gcc.target/i386/avx512vl-vec-set-2.c: New test.