[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.