Adding the StringSubstring simplified operator is a precursor to
improve inlining of String.p.{substr,substring,slice}.
This also contains a drive-by renaming to normalize different
spellings of 'Substring'.

Bug: v8:7250, v8:7340
Change-Id: I89e0fbafeab80f5d2f3ef348a5303d32c0abfe0a
Reviewed-on: https://chromium-review.googlesource.com/919084
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51522}