Introduce a dedicated StringCharCodeAt builtin, that performs the core
logic of String.prototype.charCodeAt and lower the StringCharCodeAt
simplified operator to a call to this builtin rather than inlining the
full functionality into each and every TurboFan graph using it. This can
significantly reduce compile time in some cases (i.e. can easily shave
off over 50% of compile time overhead for small functions that call
String.prototype.charCodeAt).

Currently it returns the char code as TaggedSigned value, but
middle-term we should make it possible to return untagged values
from builtins.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2600443002
Cr-Commit-Position: refs/heads/master@{#41912}