[turbofan] Relax effects and context for JSToNumber(x:plain-primitive).

[turbofan] Relax effects and context for JSToNumber(x:plain-primitive).

Relanded with fix for always returning Change for PlainPrimitive even
if there was no change. The performance regression on primes.js and
corrections.js is due to unlucky loop header alignment; will be addressed
separately.

TEST=unittests
R=svenpanne@chromium.org

Committed: https://chromium.googlesource.com/v8/v8/+/75484e8d16866eba7aa9c3b87841cd6ce2f466b8

Committed: https://crrev.com/283175d984cc3b7796d537a2b821b28e9c316c16
Cr-Commit-Position: refs/heads/master@{#25832}

[Benedikt Meurer, 2014-12-15 13:13:19]

PTAL

[Sven Panne, 2014-12-15 13:42:09]

lgtm

[Benedikt Meurer, 2014-12-15 13:43:02]

Committed patchset #1 (id:1) manually as
75484e8d16866eba7aa9c3b87841cd6ce2f466b8 (presubmit successful).

[Benedikt Meurer, 2014-12-16 04:44:41]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/806103002/ by bmeurer@chromium.org.

The reason for reverting is: Performance regressions on primes and corrections
benchmarks..

[Benedikt Meurer, 2014-12-16 05:39:54]

Committed patchset #2 (id:20001) manually as
283175d984cc3b7796d537a2b821b28e9c316c16 (presubmit successful).

[commit-bot: I haz the power, 2014-12-17 10:13:21]

Patchset 2 (id:??) landed as
https://crrev.com/75484e8d16866eba7aa9c3b87841cd6ce2f466b8
Cr-Commit-Position: refs/heads/master@{#25818}

[commit-bot: I haz the power, 2014-12-17 10:13:52]

Patchset 2 (id:??) landed as
https://crrev.com/283175d984cc3b7796d537a2b821b28e9c316c16
Cr-Commit-Position: refs/heads/master@{#25832}