Remove the duplication of the allocation logic via the
AllocateOneByteConsString and AllocateTwoByteConsString helpers, and
instead just have a diamond to figure out the result map. This reduces
code size of the StringAdd_CheckNone builtin and even seems to be
beneficial performance wise. It seems to improve the performance on
the `bench-dom-serialize.js` test by around 1% just doing this.

Drive-by-fix: Remove the `flags` from CodeStubAssembler::StringAdd()
and its helpers, since we no longer support pretenuring of string
additions (for quite a while now).

Bug: v8:8834, v8:8939
Change-Id: Ia23e02c974b5f572930fcd45be0643094ab2fa98
Doc: https://bit.ly/fast-string-concatenation-in-javascript
Reviewed-on: https://chromium-review.googlesource.com/c/1498133
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59993}