swift234

Revert "Revert "Hoist more early returns as early as possible.""

This reverts commit r25947, reinstating r25943 with fixes.

This had been preemptively reverted fearing it might have broken
test/SILPasses/devirt_value_metatypes.swift, which it did. One of the
early exits that was hoisted should not have been moved.

I plan on another clean-up for that particular early exit in a
forthcoming commit.

Swift SVN r25958