The code produced for RegExpExec is quite large, and we ended up completely
inlining it several spots.  This CL moves RegExpPrototypeExecBody into two
stubs (one each for fast and slow paths) and converts inlined uses into stub
calls. This decreases the local x64 snapshot size by around 80K.

BUG=chromium:688972

Review-Url: https://codereview.chromium.org/2677073004
Cr-Commit-Position: refs/heads/master@{#42965}