Better completion regex

[bakkot]

Regex change is necessary for tc39/ecma262#3163 to land.

The change to the regexp is

• to make it so that only "throw a" as in "throw a TypeError exception" counts, so that a reference to a "throw completion" isn't interpreted as throwing,
• to make "Return Completion Record { ... }" count as returning a completion record, and
• to distinguish between a reference to a "return completion" and a step which says to "Return Completion (...)".