VisitNewLocalBlockContext funciton was allocating two registers and checking
if they are consecutive. This cl changes it to reserve consecutive registers.
The earlier code breaks this test mjsunit/regress/regress-542099.js.

BUG=v8:4280
LOG=N

Review URL: https://codereview.chromium.org/1545003003

Cr-Commit-Position: refs/heads/master@{#33086}