We currently generate code for both separately, resulting in five
instructions (three for the i32_eqz, two for the br_if):
  test rax,rax
  seteq dl
  movzxb rdx,rdx
  test rdx,rdx
  jz <label>

After this CL, we just generate two instructions:
  test rax, rax
  jnz <label>

This is implemented by a look-ahead in the {kExprI32Eqz} handler. If the
opcode is followed by {kExprBrIf}, no code is emitted. Instead, a flag in
the {LiftoffCompiler} is set to signal to the {kExprBrIf} handler that
the previous instruction was not processed yet.
Note that this mechanism is designed to be reusable for more similar
improvements. For the single instance implemented in this CL, it is not
needed.

Plus some drive-by cleanup.

R=jkummerow@chromium.org

Bug: v8:9831
Change-Id: I47495fe763b7db7cef41aa207c88a2f1b74bf1a9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1862557
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64292}