[turbofan] Fix broken dynamic TDZ check for let and const.

[turbofan] Fix broken dynamic TDZ check for let and const.

This fixes broken dynamic hole-checks for the temporal dead zone of
non-initializing assignments to {let} and {const} declared variables.
Also note that this exemplifies a case where the dynamic check for such
assignments to {let} declared variables can no longer be elided as the
comment suggested.

R=rossberg@chromium.org
TEST=mjsunit/regress/regress-4388
BUG=v8:4388
LOG=n

Review URL: https://codereview.chromium.org/1318693002

Cr-Commit-Position: refs/heads/master@{#30375}