AArch64: Introduce a DAG combine for folding offsets into addresses.

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: 329956