[PATCH] RISC-V: Support simplify (-1-x) for vector.

From: Yanzhang Wang <yanzhang.wang@intel.com>

The pattern is enabled for scalar but not for vector. The patch try to
make it consistent and will convert below code,

shortcut_for_riscv_vrsub_case_1_32:
        vl1re32.v       v1,0(a1)
        vsetvli zero,a2,e32,m1,ta,ma
        vrsub.vi        v1,v1,-1
        vs1r.v  v1,0(a0)
        ret

to,

shortcut_for_riscv_vrsub_case_1_32:
        vl1re32.v       v1,0(a1)
        vsetvli zero,a2,e32,m1,ta,ma
        vnot.v  v1,v1
        vs1r.v  v1,0(a0)
        ret

gcc/ChangeLog:

	* simplify-rtx.cc (simplify_context::simplify_binary_operation_1): Use
	CONSTM1_RTX.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/simplify-vrsub.c: New test.