[x86] Several small performance improvements.

[x86] Several small performance improvements.

- Generate

    cmp reg, 0

  instead of

    test reg, -1

  for word tests.
- Generate

    mov reg, [stack slot]
    add reg, reg
    jo label

  instead of

    mov reg, [stack slot]
    add reg, [stack slot]
    jo label

  for SMI tagging.
- Improve materialization of int32 constants on X86-64.
- Branch fusion fix for Word64And and Int64Sub on X86-64.

R=dcarney@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=24619

[Benedikt Meurer, 2014-10-15 08:59:24]

PTAL

[dcarney, 2014-10-15 09:01:27]

lgtm

https://codereview.chromium.org/651383003/diff/1/src/compiler/x64/code-genera...
File src/compiler/x64/code-generator-x64.cc (left):

https://codereview.chromium.org/651383003/diff/1/src/compiler/x64/code-genera...
src/compiler/x64/code-generator-x64.cc:872: // TODO(dcarney): don't need scratch
in this case.
this comment still holds

[Benedikt Meurer, 2014-10-15 09:05:49]

Committed patchset #2 (id:20001) manually as 24619 (presubmit successful).