[MemCpyOpt] fix miscompile for non-dominated use of src alloca for stack-move optimization (#66618)

Stack-move optimization, the optimization that merges src and dest
alloca of the full-size copy, replaces all uses of the dest alloca with
src alloca. For safety, we needed to check all uses of the dest alloca
locations are dominated by src alloca, to be replaced. This PR adds the
check for that.

Fixes #65225