We still avoid the "then" lookup using the current fast-path
mega-guard in the baseline case, but in TurboFan we simply
constant-fold the "then" lookup in the JSCallReducer. So all
further optimizations on Promise#then in TurboFan will automatically
apply to Promise#catch as well.

Bug: v8:7253
Change-Id: Idf7252157375a0ae3a91c7a3b42c30c5f367c0a8
Reviewed-on: https://chromium-review.googlesource.com/895446
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51008}