SROA: Handle a case of store size being smaller than allocation size

In this case, we are creating an x86_fp80 slice for a union from C where
the padding bytes may contain real data. An x86_fp80 alloca is 16 bytes,
and that's just fine. We can't, however, use regular loads and stores to
access the slice, because the store size is only 10 bytes / 80 bits.
Instead, use memcpy and memset.

Fixes PR18726.

Reviewed By: chandlerc

Differential Revision: http://reviews.llvm.org/D5012

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216248 91177308-0d34-0410-b5e6-96231b3b80d8