[turbofan] Fix exit control flow in TryCatchBuilder.

[turbofan] Fix exit control flow in TryCatchBuilder.

This makes sure that the exit control flow that merges the try-block
with the catch-block after a try-catch-statement creates a new merge
node in cases where it has to. Otherwise dangling phi nodes might have
the wrong number of value inputs.

R=bmeurer@chromium.org
TEST=mjsunit/regress/regress-crbug-505354
BUG=chromium:505354
LOG=N

Review URL: https://codereview.chromium.org/1213183003

Cr-Commit-Position: refs/heads/master@{#29362}