This patch improves DAGCombiner's handling of potential store merges by
detecting function calls between loads and stores. When a function call
exists in the chain between a load and its corresponding store, we avoid
merging these stores if the spilling is unprofitable.
We had to implement a hook on TLI, since TTI is unavailable in
DAGCombine. Currently, it's only enabled for riscv.
This is the DAG equivalent of PR #129258
f138e36d52