[SROA] Simplify the computing of alignment: we only ever need the
alignment of the slice being rewritten, not any arbitrary offset.

Every caller is really just trying to compute the alignment for the
whole slice, never for some arbitrary alignment. They are also just
passing a type when they have one to see if we can skip an explicit
alignment in the IR by using the type's alignment. This makes for a much
simpler interface.

Another refactoring inspired by the addrspace patch for SROA, although
only loosely related.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202230 91177308-0d34-0410-b5e6-96231b3b80d8
[SROA] Yet another slight refactoring that simplifies an API in the
rewriting logic: don't pass custom offsets for the adjusted pointer to
the new alloca.

We always passed NewBeginOffset here. Sometimes we spelled it
BeginOffset, but only when they were in fact equal. Whats worse, the API
is set up so that you can't reasonably call it with anything else -- it
assumes that you're passing it an offset relative to the *original*
alloca that happens to fall within the new one. That's the whole point
of NewBeginOffset, it's the clamped beginning offset.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202231 91177308-0d34-0410-b5e6-96231b3b80d8
[SROA] The original refactoring inspired by the addrspace patch in
D1764, which in turn set off the other refactorings to make
'getSliceAlign()' a sensible thing.

There are two possible inputs to the required alignment of a memory
transfer intrinsic: the alignment constraints of the source and the
destination. If we are *only* introducing a (potentially new) offset
onto one side of the transfer, we don't need to consider the alignment
constraints of the other side. Use this to simplify the logic feeding
into alignment computation for unsplit transfers.

Also, hoist the clamp of the magical zero alignment for these intrinsics
to the more customary one alignment early. This lets several other
conditions melt away.

No functionality changed. There is a further improvement this exposes
which *will* change functionality, but that's arriving in a separate
patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202232 91177308-0d34-0410-b5e6-96231b3b80d8