RISC-V: FAIL:g++.dg/torture/vshuf-v[2|4]di.C -Os (execution test) on RV32

RISC-V: FAIL:g++.dg/torture/vshuf-v[2|4]di.C -Os (execution test) on RV32

This patch fixs the issue of g++.dg/torture/vshuf-v2di.C
and g++.dg/torture/vshuf-v4di.C -Os execution failure with
-march=rv32gcv -mabi=ilp32d.

Consider the following code:
typedef unsigned long long V __attribute__((vector_size(16)));

.LC0: 0xc1c2c3c4c5c6c7c8

before this patch:

lui	a5,%hi(.LC0)
addi	a5,a5,%lo(.LC0)
lw	a6,4(a5)//0xc1c2c3c4
lw	a5,0(a5)//0xc5c6c7c8
vsetivli	zero,2,e64,m1,ta,mu
vmv.v.x	v2,a5//v2 is {0xffffffffc5c6c7c8, 0xffffffffc5c6c7c8}

after this patch:

lui	a5,%hi(.LC0)
addi	a5,a5,%lo(.LC0)
vsetivli	zero,2,e64,m1,ta,mu
vlse64.v	v2,0(a5),zero//v2 is {0xc1c2c3c4c5c6c7c8, 0xc1c2c3c4c5c6c7c8}

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (sew64_scalar_helper): Bugfix.