Reland r329956, "AArch64: Introduce a DAG combine for folding offsets into addresses...

Reland r329956, "AArch64: Introduce a DAG combine for folding offsets into addresses.", with a fix for the bot failure.

This reland includes a check to prevent the DAG combiner from folding an
offset that is smaller than the existing one. This can cause oscillations
between two possible DAGs, which was the cause of the hang and later assertion
failure observed on the lnt-ctmark-aarch64-O3-flto bot.
http://green.lab.llvm.org/green/job/lnt-ctmark-aarch64-O3-flto/2024/

Original commit message:
> This is a code size win in code that takes offseted addresses
> frequently, such as C++ constructors that typically need to compute
> an offseted address of a vtable. This reduces the size of Chromium
> for Android's .text section by 108KB.

Differential Revision: https://reviews.llvm.org/D45199

llvm-svn: 330630