SROA: The alloca type isn't a candidate promotion type for vectors

The alloca's type is irrelevant, only those types which are used in a
load or store of the exact size of the slice should be considered.

This manifested as an assertion failure when we compared the various
types: we had a size mismatch.

This fixes PR21480.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222499 91177308-0d34-0410-b5e6-96231b3b80d8
This Reassociate change unintentionally slipped in r222499

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222500 91177308-0d34-0410-b5e6-96231b3b80d8
Add accessor marcos to ConstantPlaceHolder, similar to those in the base class.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222502 91177308-0d34-0410-b5e6-96231b3b80d8