RISC-V: Fix bug of VLA SLP auto-vectorization

Sorry for producing bugs in the previous VLA SLP patch.

Consider this following permutation:
_85 = VEC_PERM_EXPR <{ 99, 17, ... }, { 11, 80, ... }, { 0, POLY_INT_CST [4, 4], 1, POLY_INT_CST [5, 4], 2, POLY_INT_CST [6, 4], ... }>;

The correct result should be:
_85 = { 99, 11, 17, 80, ... }

However, I did wrong in the previous patch.

Code sequence before this patch:

set mask = { 0, 1, 0, 1, ... }
set v0 = { 99, 17, 99, 17, ... }
set v1 = { 11, 80, 11, 80, ... }
set index = viota (mask) = { 0, 0, 1, 1, 2, 2, ... }
set result = vrgather_mu (v0, v1, index, mask) = { 99, 11, 99, 80 }
The result is incorrect.

After this patch:

set mask = { 0, 1, 0, 1, ... }
set index = viota (mask) = { 0, 0, 1, 1, 2, 2, ... }
set v0 = vrgather ({ 99, 17, 99, 17, ... }, index) = { 99, 99, 17, 17, ... }
set v1 = { 11, 80, 11, 80, ... }
set result = vrgather_mu (v0, v1, index, mask) = { 99, 11, 17, 80 }
The result is what we expected.

This issue was discovered in the test I appended in this patch with --param=riscv-autovec-lmul=2.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (emit_vlmax_decompress_insn): Fix bug.
	(shuffle_decompress_patterns): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/partial/slp-12.c: New test.
	* gcc.target/riscv/rvv/autovec/partial/slp_run-12.c: New test.