This is the initial step towards refactoring the regexp literation
creation code to make it less obscure and more similar to the mechanism
we use to create array and object literals.  There's now a new runtime
entry %CreateRegExpLiteral with the same interface as the entries for
array and object literals, except that we still pass the flags as
string.

Instead of embedding the hand written native to clone JSRegExp instances
we now have a FastCloneRegExpStub, which behaves similar to the other
FastCloneShallowArrayStub and FastCloneShallowObjectStub that we already
had.

R=mlippautz@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/1475823003

Cr-Commit-Position: refs/heads/master@{#32255}