rearrange how SRoA handles promotion of allocas to vectors.

rearrange how SRoA handles promotion of allocas to vectors.
With the new world order, it can handle cases where the first
store into the alloca is an element of the vector, instead of
requiring the first analyzed store to have the vector type
itself.  This allows us to un-xfail
test/CodeGen/X86/vec_ins_extract.ll.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63590 91177308-0d34-0410-b5e6-96231b3b80d8