[ARM] Do not test for CPUs, use SubtargetFeatures. Also remove 2 flags.

This is a follow-up for r273544.

The end goal is to get rid of the isSwift / isCortexXY / isWhatever methods.

This commit also removes two command-line flags that weren't used in any of the
tests: widen-vmovs and swift-partial-update-clearance. The former may be easily
replaced with the mattr mechanism, but the latter may not (as it is a subtarget
property, and not a proper feature).

Differential Revision: http://reviews.llvm.org/D21797

llvm-svn: 274620