[AArch64] Implements the lowering of formal arguments for GlobalISel.

This is just a trivial implementation:
- Support only arguments passed in registers.
- Support only "plain" arguments, i.e., no sext/zext attribute.

At this point, it is possible to play with the IRTranslator on AArch64:
llc -mtriple arm64-<vendor>-<os> -print-machineinstrs <input.ll> -o - -global-isel

For now, we only support the translation of program with adds and returns.

Follow-up patches are on their way to add a test case (the MIRParser is
not ready as it is).

llvm-svn: 260600