[AArch64] Switch regression tests to test features not CPUs

Summary:
We have switched to using features for all heuristics, but
the tests for these are still using -mcpu, which means we
are not directly testing the features.

This converts at least some of the existing regression tests
to use the new features.

This still leaves the following features untested:

merge-narrow-ld
predictable-select-expensive
alternate-sextload-cvt-f32-pattern
disable-latency-sched-heuristic

Reviewers: mcrosier, t.p.northover, rengolin

Subscribers: MatzeB, aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D21288

llvm-svn: 273271