RISC-V: Robostify shuffle index used by vrgather and fix regression

RISC-V: Robostify shuffle index used by vrgather and fix regression

Notice there are some regression FAILs:
FAIL: gcc.target/riscv/rvv/autovec/pr110950.c -O3 -ftree-vectorize  scan-assembler-times vslide1up\\.vx 1
FAIL: gcc.target/riscv/rvv/autovec/vls-vlmax/perm-4.c -std=c99 -O3 -ftree-vectorize --param riscv-autovec-preference=fixed-vlmax  scan-assembler-times vrgather\\.vv\\tv[0-9]+,\\s*v[0-9]+,\\s*v[0-9]+ 19
FAIL: gcc.target/riscv/rvv/autovec/vls-vlmax/perm-4.c -std=c99 -O3 -ftree-vectorize --param riscv-autovec-preference=fixed-vlmax  scan-assembler-times vrgatherei16\\.vv\\tv[0-9]+,\\s*v[0-9]+,\\s*v[0-9]+ 12
FAIL: gcc.target/riscv/rvv/autovec/vls/perm-4.c -O3 -ftree-vectorize --param riscv-autovec-preference=scalable  scan-assembler-times vrgather\\.vv\\tv[0-9]+,\\s*v[0-9]+,\\s*v[0-9]+ 19
FAIL: gcc.target/riscv/rvv/autovec/vls/perm-4.c -O3 -ftree-vectorize --param riscv-autovec-preference=scalable  scan-assembler-times vrgatherei16\\.vv\\tv[0-9]+,\\s*v[0-9]+,\\s*v[0-9]+ 12

pr110950 is not a regression, adapt testcase is enough.

The rest FAILs which is caused by this patch:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=d9dd06ad51b7479f09acb88adf404664a1e18b2a

need to be recovered back.

Robostify the gather index to fixe those FAILs.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (get_gather_index_mode): New function.
	(shuffle_series_patterns): Robostify shuffle index.
	(shuffle_generic_patterns): Ditto.

gcc/testsuite/ChangeLog:

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