RISC-V: Optimize fault only first load

RISC-V: Optimize fault only first load

V2 patch for: https://patchwork.sourceware.org/project/gcc/patch/20230330012804.110539-1-juzhe.zhong@rivai.ai/
which has been reviewed.

This patch address Jeff's comment, refine ChangeLog to give more
clear information.

gcc/ChangeLog:

	* config/riscv/vector-iterators.md: New unspec to refine fault first load pattern.
	* config/riscv/vector.md: Refine fault first load pattern to erase avl from instructions
	with the fault first load property.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/vsetvl/ffload-1.c: New test.
	* gcc.target/riscv/rvv/vsetvl/ffload-2.c: New test.
	* gcc.target/riscv/rvv/vsetvl/ffload-3.c: New test.
	* gcc.target/riscv/rvv/vsetvl/ffload-5.c: New test.
	* gcc.target/riscv/rvv/vsetvl/ffload-6.c: New test.
	* gcc.target/riscv/rvv/vsetvl/ffload-7.c: New test.