RISC-V: Disable small memsets for xtheadvector [PR123910].

RISC-V: Disable small memsets for xtheadvector [PR123910].

This patch disables memsets with size less than a vector for
xtheadvector.  As xtheadvector does not support fractional
LMUL we need to ensure to not emit those vectors that might
use it.

	PR target/123910

gcc/ChangeLog:

	* config/riscv/riscv-string.cc (riscv_expand_block_move):
	Remove !xtheadvector guard.
	(use_vector_stringop_p): Guard small LMULs.
	(check_vectorise_memory_operation): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr123910.c: New test.
	* gcc.target/riscv/rvv/xtheadvector/pr114194-rv32.c: xfail.

Signed-off-by: Robin Dapp <rdapp@oss.qualcomm.com>