Revert "[MemDep][NFCI] Remove redundant dyn_cast, replace with cast"

This reverts commit 180d3f25.  This commit is simply wrong.  IsLoad is set within the same file based on modref state, not whether the instruction is a LoadInst.

This went uncaught because cast<Ty>(X) has been broken.  See https://discourse.llvm.org/t/cast-x-is-broken-implications-and-proposal-to-address/63033 for context.