[PATCH] RISC-V: Fix slide pattern recognition [PR122124]

[PATCH] RISC-V: Fix slide pattern recognition [PR122124]

Ensure the second pivot is really a pivot and it's not in OP1.

	PR target/122124
gcc/ChangeLog:
	* config/riscv/riscv-v.cc (shuffle_slide_patterns): Check if
	the second pivot is in OP1 and improve comments.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/rvv/autovec/pr122124.c: New test.