RISC-V: Check if we can vec_extract [PR121510].

For Zvfhmin a vector mode exists but the corresponding vec_extract does
not.  This patch checks that a vec_extract is available and otherwise
falls back to standard handling.

	PR target/121510

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_legitimize_move): Check if we can
	vec_extract.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr121510.c: New test.

(cherry picked from commit a6bf07653cd272add46a2218ec141c95d7f02427)