[PATCH v2 3/3] RISC-V: cmpmem for RISCV with V extension

[PATCH v2 3/3] RISC-V: cmpmem for RISCV with V extension

So this is the cmpmem patch from Sergei, updated for the trunk.

Updates included adjusting the existing cmpmemsi expander to
conditionally try expansion via vector.  And a minor testsuite
adjustment to turn off vector expansion in one test that is primarily
focused on vset optimization and ensuring we don't have extras.

I've spun this in my tester successfully and just want to see a clean
run through precommit CI before moving forward.

Jeff
gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_vector::expand_vec_cmpmem): New
	function declaration.
	* config/riscv/riscv-string.cc (riscv_vector::expand_vec_cmpmem): New
	function.
	* config/riscv/riscv.md (cmpmemsi): Try riscv_vector::expand_vec_cmpmem
	for constant lengths.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/cmpmem-1.c: New codegen tests
	* gcc.target/riscv/rvv/base/cmpmem-2.c: New execution tests
	* gcc.target/riscv/rvv/base/cmpmem-3.c: New codegen tests
	* gcc.target/riscv/rvv/base/cmpmem-4.c: New codegen tests
	* gcc.target/riscv/rvv/autovec/vls/misalign-1.c: Turn off vector mem* and
	str* handling.