Each call to emit_32 uses 5 constant pool slots:
* the "emit_32" string
* undefined (the receiver)
* the argument (heap number)
* the load IC
* the call IC

This change cuts that down 20% to 4, by loading the undefined from the heap roots.

R=verwaest@chromium.org
BUG=

Review URL: https://codereview.chromium.org/980563002

Cr-Commit-Position: refs/heads/master@{#26979}