CpuFeatures::IsSupported(feature) indicates that the feature is
available on the target. AssemblerBase::IsEnabled(feature) indicates
that we've checked for support (using CpuFeatureScope). The main benefit
is that we can test on (for example) ARMv8, but have some assurance that
we won't generate ARMv8 instructions on ARMv7 targets.

This patch simply cleans up the usage, which had become inconsistent.
The instruction emission functions now check not only that their
dependent features are supported, but also that we've verified that
using CpuFeatureScope.

BUG=

Review-Url: https://codereview.chromium.org/2360243002
Cr-Commit-Position: refs/heads/master@{#39676}