[AArch64] Use 16 bytes as preferred function alignment on Cortex-A72.

Summary:
This change gives a 0.34% speed on execution time, a 0.61% improvement
in benchmark scores and a 0.57% increase in binary size on a Cortex-A72.
These numbers are the geomean results on a wide range of benchmarks from
the test-suite, SPEC2000, SPEC2006 and a range of proprietary suites.

The software optimization guide for the Cortex-A72 recommends 16 byte
branch alignment.


Reviewers: t.p.northover, kristof.beyls, rengolin, sbaranga, mcrosier, javed.absar

Reviewed By: kristof.beyls

Subscribers: llvm-commits, aemerson

Differential Revision: https://reviews.llvm.org/D34961

llvm-svn: 307380