The instruction selector currently sometimes emits a lea32 with an
offset of 0, which the code generator just ignores (emits no code at
all). This can result in the result of TruncateInt64ToInt32 to not be
zero extended.
This CL fixes that by disallowing lea32 instructions with 0 offset, and
fixing the instruction selector to generate a movl or just no code for
that case.

R=jarin@chromium.org

Bug: chromium:863810, v8:7947
Change-Id: I1b21fc5f0fda9ca3144917538c3d0bbf46601c33
Reviewed-on: https://chromium-review.googlesource.com/1137825Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54489}