[DAGCombiner][RISCV] Return SDValue(N, 0) instead of SDValue() after 2 calls...

[DAGCombiner][RISCV] Return SDValue(N, 0) instead of SDValue() after 2 calls to CombineTo in visitSTORE.

RISC-V found a case where the CombineTo caused N to be CSEd with
an existing node and then deleted. The top level DAGCombiner loop
was surprised to find a node was deleted, but SDValue() was returned
from the visit function.

We need to return SDValue(N, 0) to tell the top level loop that
a change was made, but the worklist updates were already handled.

Fixes #64772.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D158208