[RISC-V] Fix minor testsuite fallout from recent hoisting change

This change from Richi:

> commit 66ed104db50c639c79e7ba2826ccd6df41cb37c1
> Author: Richard Biener <rguenther@suse.de>
> Date:   Mon Aug 24 11:24:57 2026 +0200
>
>    tree-optimization/126997 - hoist invariants after unswitching
>

Slightly improves the code for the pr123501 testcase in the RISC-V testsuite.
This patch adjusts the expected output for the improved code.  In particular we
only have a single vcpop.m now.

Pushing to the trunk.

gcc/testsuite
	* gcc.target/riscv/rvv/autovec/reduc/pr123501.c: Adjust expected
	output.