Now that SROA can form alloca's for dynamic vector accesses, further improve...

Now that SROA can form alloca's for dynamic vector accesses, further improve it to be able to replace operations on these vector alloca's with insert/extract element insts

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158623 91177308-0d34-0410-b5e6-96231b3b80d8