mjsunit/es6/mirror-collections fails with ignition because dead registers
may hold references to objects. This prevents GC from collecting
otherwise dead objects. Dead registers are not cleared because the cost
of clearing them outweighs its benefits. Hence, modifying this test to
work around this problem.

BUG=v8:4280,v8:4853
LOG=N

Review-Url: https://codereview.chromium.org/1945223002
Cr-Commit-Position: refs/heads/master@{#36098}