While scanning through the uses of an alloca, keep track of the current offset
relative to the start of the alloca, and check memory references to see if
the offset & size correspond to a component within the alloca.  This has the
nice benefit of unifying much of the code from isSafeUseOfAllocation,
isSafeElementUse, and isSafeUseOfBitCastedAllocation.  The code to rewrite
the uses of a promoted alloca, after it is determined to be safe, is
reorganized in the same way.

Also, when rewriting GEP instructions, mark them as "in-bounds" since all the
indices are known to be safe.

llvm-svn: 91184