[SROA] Use NewOffsetBegin in the unsplit case for memset merely for consistency with memcpy rewriting, and fix a latent bug in the alignment management for memset.
[SROA] Use NewOffsetBegin in the unsplit case for memset merely for
consistency with memcpy rewriting, and fix a latent bug in the alignment
management for memset.

The alignment issue is that getAdjustedAllocaPtr is computing the
*relative* offset into the new alloca, but the alignment isn't being set
to the relative offset, it was using the the absolute offset which is
into the old alloca.

I don't think its possible to write a test case that actually reaches
this code where the resulting alignment would be observably different,
but the intent was clearly to use the relative offset within the new
alloca.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@202229 91177308-0d34-0410-b5e6-96231b3b80d8

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