Use a NumberConstant op rather than an Int32Constant to make assertions in
v8::internal::compiler::EscapeAnalysis::ProcessAllocation() happy.

BUG=v8:5598
R=bmeurer@chromium.org, franzih@chromium.org

Review-Url: https://codereview.chromium.org/2478643002
Cr-Commit-Position: refs/heads/master@{#40746}