Make FlushICache NOP for Nvidia Denver CPU's.

Denver supports a coherent cache mechanism. There is no need to clean
the D cache and invalidate I cache. MTS has to check the translation
anytime there is an I cache invalidate and this time can be saved by
making FlushICache a NOP.

The patch improves Octane by roughly 3-4% on Denver.

Review URL: https://codereview.chromium.org/797233002

Cr-Commit-Position: refs/heads/master@{#25898}
[mips] Fix typos introduced by previous commit.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/814203002

Cr-Commit-Position: refs/heads/master@{#25899}
Improved --trace-opt output a bit.

This ensures that we always have a kind of "brace" around the actual compilation
with --trace-opt. Previously this was coupled to --trace-hydrogen for
Crankshaft, and there was no output at the start of the compilation for
TurboFan.

Removed redundant "[completed...]" output: Whenever the compilation was
successful, we have already printed "[optimizing...took...]".

Output total TurboFanning time with --trace-opt, too (basically as graph
building time, this is rather arbitrary).

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/819613002

Cr-Commit-Position: refs/heads/master@{#25900}