[RISCV] Avoid infinite loop between DAGCombiner::visitMUL and...

[RISCV] Avoid infinite loop between DAGCombiner::visitMUL and RISCVISelLowering::transformAddImmMulImm

See https://github.com/llvm/llvm-project/issues/53831 for a full discussion.

The basic issue is that DAGCombiner::visitMUL and
RISCVISelLowering;:transformAddImmMullImm get stuck in a loop, as the
current checks in transformAddImmMulImm aren't sufficient to avoid all
cases where DAGCombiner::isMulAddWithConstProfitable might trigger a
transformation. This patch makes transformAddImmMulImm bail out if C0
(the constant used for multiplication) has more than one use.

Differential Revision: https://reviews.llvm.org/D120332