[DAGCombine] MergeConsecutiveStores - fix cppcheck/MSVC extension warning. NFCI.

Set the StartIdx type to size_t so that it matches the StoreNodes SmallVector size() and index types.

Silences the MSVC analyzer warning that unsigned increment might overflow before exceeding size_t on 64-bit targets - this isn't likely to happen but it means we use consistent types and reduces the warning "noise" a little.

llvm-svn: 368998