RISC-V: Bugfix for scalar move with merged operand

Given below example for VLS mode

void
test (vl_t *u)
{
  vl_t t;
  long long *p = (long long *)&t;

  p[0] = p[1] = 2;

  *u = t;
}

The vec_set will simplify the insn to vmv.s.x when index is 0, without
merged operand. That will result in some problems in DCE, aka:

1:  137[DI] = a0
2:  138[V2DI] = 134[V2DI]                              // deleted by DCE
3:  139[DI] = #2                                       // deleted by DCE
4:  140[DI] = #2                                       // deleted by DCE
5:  141[V2DI] = vec_dup:V2DI (139[DI])                 // deleted by DCE
6:  138[V2DI] = vslideup_imm (138[V2DI], 141[V2DI], 1) // deleted by DCE
7:  135[V2DI] = 138[V2DI]                              // deleted by DCE
8:  142[V2DI] = 135[V2DI]                              // deleted by DCE
9:  143[DI] = #2
10: 142[V2DI] = vec_dup:V2DI (143[DI])
11: (137[DI]) = 142[V2DI]

The higher 64 bits of 142[V2DI] is unknown here and it generated incorrect
code when store back to memory. This patch would like to fix this issue
by adding a new SCALAR_MOVE_MERGED_OP for vec_set.

Please note this patch doesn't enable VLS for vec_set, the underlying
patches will support this soon.

gcc/ChangeLog:

	* config/riscv/autovec.md: Bugfix.
	* config/riscv/riscv-protos.h (SCALAR_MOVE_MERGED_OP): New enum.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/scalar-move-merged-run-1.c: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>