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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368998 91177308-0d34-0410-b5e6-96231b3b80d8