CSA builtins can become very large, and the RegExp builtins are currently the
main offender (e.g. @@match's code size is over 50k). This is due to the fact
that most RegExp builtins rely on RegExpBuiltinExec (fairly large itself),
which is then inlined multiple times in many builtins.

This CL reduces the snapshot size for an x64 release build by 80k by turning
slow-path RegExpBuiltinExec calls into stub calls (i.e. removing code
duplication through inlining) and completely removing the code path for fast
RegExp instances in RegExpExec (it is never taken).

BUG=v8:5339,v8:5737

Review-Url: https://codereview.chromium.org/2745053003
Cr-Commit-Position: refs/heads/master@{#43889}