Consistently use only one of virtual/OVERRIDE/FINAL.

FINAL implies OVERRIDE, which in turn implies virtual, so there's no need to use
more than one of these. The Google C++ style guide even requires this, see
http://google-styleguide.googlecode.com/svn/trunk/cppguide.html#Inheritance.

While we're here, port r24662 to x87.

The net result is that v8 compiles again with a current clang.

BUG=v8:3753
LOG=y

Review URL: https://codereview.chromium.org/797943002

Cr-Commit-Position: refs/heads/master@{#25792}
[turbofan] Quickfix for invalid number truncation of typed array loads.

TEST=mjsunit/compiler/regress-int32array-outofbounds-nan
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/803483002

Cr-Commit-Position: refs/heads/master@{#25793}