Provide convenient access to the zext/sext attributes of function arguments. NFC.
[FastISel][AArch64] Don't perform sign-/zero-extension for function arguments that have already been sign-/zero-extended.
Revert "r214832 - MachineCombiner Pass for selecting faster instruction"

It broke compiling of most Benchmark and internal test, as clang got
clashed by segmentation fault or assertion.
[FastISel][AArch64] Implement the FastLowerArguments hook.

This implements basic argument lowering for AArch64 in FastISel. It only
handles a small subset of the C calling convention. It supports simple
arguments that can be passed in GPR and FPR registers.

This should cover most of the trivial cases without falling back to
SelectionDAG.

This fixes <rdar://problem/17890986>.