Don't drop the alignment on a memcpy intrinsic when producing a store. This is

Don't drop the alignment on a memcpy intrinsic when producing a store. This is
only a missed optimization opportunity if the store is over-aligned, but a
miscompile if the store's new type has a higher natural alignment than the
memcpy did. Fixes PR13920!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164641 91177308-0d34-0410-b5e6-96231b3b80d8