[turbofan] Preserve cached code across GCs.

[turbofan] Preserve cached code across GCs.

This preserves the context-independent entry in an optimized code map
across GCs when the code is considered young (i.e. less than 3 ages).
Note that any context-dependent entry for the same code will still be
flushed immediately when the respective context dies, hence context
lifetime is not increased.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1252463002

Cr-Commit-Position: refs/heads/master@{#29790}