Revert of X87: disable the regress-undefined-nan test case for x87. (patchset...

Revert of X87: disable the regress-undefined-nan test case for x87. (patchset #1 id:1 of https://codereview.chromium.org/1414733004/ )

Reason for revert:
because of merge mistake, "regress/" is missed when skipping one test case for X87.

 "regress/" will be added when relanding it.

Original issue's description:
> X87: disable the regress-undefined-nan test case for x87.
>
>   The reason is when native_context_specialization flag is ture, X87 turbofan
>   will hit the known issue that X87 will change a sNaN to qNaN by default. And
>   then it will fail when bit-comparing the source (sNaN) and the result (qNaN).
>
> BUG=
>
> Committed: https://crrev.com/b3c719ebbad6c87afefa33a7d0b3f412b2e304db
> Cr-Commit-Position: refs/heads/master@{#31530}

TBR=bmeurer@chromium.org,weiliang.lin@intel.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1417303005

Cr-Commit-Position: refs/heads/master@{#31543}

test/mjsunit/mjsunit.status

@@ -576,12 +576,6 @@
   'mirror-script': [PASS, NO_VARIANTS],
 }],  # 'arch == mips'
 
-##############################################################################
-['arch == x87', {
-  # Turbofan will hit the known issue that x87 changes sNaN to qNaN by default.
-  'regress-undefined-nan': [SKIP],
-}],  # 'arch == x87'
-
 ##############################################################################
 ['arch == mips64el or arch == mips64', {