RISC-V: Support highpart overlap for indexed load with SRC EEW < DEST EEW

RISC-V: Support highpart overlap for indexed load with SRC EEW < DEST EEW

Leverage previous approach.

Before this patch:

.L5:
        add     a3,s0,s2
        add     a4,s6,s2
        add     a5,s7,s2
        vsetvli zero,s0,e64,m8,ta,ma
        vle8.v  v4,0(s2)
        vle8.v  v3,0(a3)
        mv      s2,s1
        vle8.v  v2,0(a4)
        vle8.v  v1,0(a5)
        nop
        vluxei8.v       v8,(s1),v4
        vs8r.v  v8,0(sp)              ---> spill
        vluxei8.v       v8,(s1),v3
        vluxei8.v       v16,(s1),v2
        vluxei8.v       v24,(s1),v1
        nop
        vmv.x.s a1,v8
        vl8re64.v       v8,0(sp)     ---> reload
        vmv.x.s a3,v24
        vmv.x.s a2,v16
        vmv.x.s a0,v8
        add     s1,s1,s5
        call    sumation
        add     s3,s3,a0
        bgeu    s4,s1,.L5

After this patch:

.L5:
	add	a3,s0,s2
	add	a4,s6,s2
	add	a5,s7,s2
	vsetvli	zero,s0,e64,m8,ta,ma
	vle8.v	v15,0(s2)
	vle8.v	v23,0(a3)
	mv	s2,s1
	vle8.v	v31,0(a4)
	vle8.v	v7,0(a5)
	vluxei8.v	v8,(s1),v15
	vluxei8.v	v16,(s1),v23
	vluxei8.v	v24,(s1),v31
	vluxei8.v	v0,(s1),v7
	vmv.x.s	a3,v0
	vmv.x.s	a2,v24
	vmv.x.s	a1,v16
	vmv.x.s	a0,v8
	add	s1,s1,s5
	call	sumation
	add	s3,s3,a0
	bgeu	s4,s1,.L5

	PR target/112431

gcc/ChangeLog:

	* config/riscv/vector.md: Support highpart overlap for indexed load.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr112431-28.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-29.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-30.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-31.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-32.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-33.c: New test.