RISC-V: Support highpart register overlap for widen vx/vf instructions

RISC-V: Support highpart register overlap for widen vx/vf instructions

This patch leverages the same approach as vwcvt.

Before this patch:

.L5:
        add     a3,s0,s1
        add     a4,s6,s1
        add     a5,s7,s1
        vsetvli zero,s0,e32,m4,ta,ma
        vle32.v v16,0(s1)
        vle32.v v12,0(a3)
        mv      s1,s2
        vle32.v v8,0(a4)
        vle32.v v4,0(a5)
        nop
        vfwadd.vf       v24,v16,fs0
        vfwadd.vf       v16,v12,fs0
        vs8r.v  v16,0(sp)                -----> spill
        vfwadd.vf       v16,v8,fs0
        vfwadd.vf       v8,v4,fs0
        nop
        vsetvli zero,zero,e64,m8,ta,ma
        vfmv.f.s        fa4,v24
        vl8re64.v       v24,0(sp)       -----> reload
        vfmv.f.s        fa5,v24
        fcvt.lu.d a0,fa4,rtz
        fcvt.lu.d a1,fa5,rtz
        vfmv.f.s        fa4,v16
        vfmv.f.s        fa5,v8
        fcvt.lu.d a2,fa4,rtz
        fcvt.lu.d a3,fa5,rtz
        add     s2,s2,s5
        call    sumation
        add     s3,s3,a0
        bgeu    s4,s2,.L5

After this patch:

.L5:
	add	a3,s0,s1
	add	a4,s6,s1
	add	a5,s7,s1
	vsetvli	zero,s0,e32,m4,ta,ma
	vle32.v	v4,0(s1)
	vle32.v	v28,0(a3)
	mv	s1,s2
	vle32.v	v20,0(a4)
	vle32.v	v12,0(a5)
	vfwadd.vf	v0,v4,fs0
	vfwadd.vf	v24,v28,fs0
	vfwadd.vf	v16,v20,fs0
	vfwadd.vf	v8,v12,fs0
	vsetvli	zero,zero,e64,m8,ta,ma
	vfmv.f.s	fa4,v0
	vfmv.f.s	fa5,v24
	fcvt.lu.d a0,fa4,rtz
	fcvt.lu.d a1,fa5,rtz
	vfmv.f.s	fa4,v16
	vfmv.f.s	fa5,v8
	fcvt.lu.d a2,fa4,rtz
	fcvt.lu.d a3,fa5,rtz
	add	s2,s2,s5
	call	sumation
	add	s3,s3,a0
	bgeu	s4,s2,.L5

	PR target/112431

gcc/ChangeLog:

	* config/riscv/vector.md: Support highpart overlap for vx/vf.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr112431-22.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-23.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-24.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-25.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-26.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-27.c: New test.