RISC-V: Support NPATTERNS = 1 stepped vector[PR110950]

RISC-V: Support NPATTERNS = 1 stepped vector[PR110950]

This patch fix ICE: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110950

0x1cf8939 expand_const_vector
        ../../../riscv-gcc/gcc/config/riscv/riscv-v.cc:1587

	PR target/110950

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_const_vector): Add NPATTERNS = 1
	stepped vector support.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr110950.c: New test.