Revert of Use -fno-delete-null-pointer-checks with gcc builds (patchset #2...

Revert of Use -fno-delete-null-pointer-checks with gcc builds (patchset #2 id:20001 of https://codereview.chromium.org/2310513002/ )

Reason for revert:
Fails on MIPS: https://build.chromium.org/p/client.v8.ports/builders/V8%20Mips%20-%20builder/builds/3653

Original issue's description:
> Use -fno-delete-null-pointer-checks with gcc builds
>
> R=bmeurer@chromium.org, jochen@chromium.org, machenbach@chromium.org
> BUG=v8:3782
>
> Committed: https://crrev.com/dbefc8ee2e9ee6e41b83f3d09c788c34bc923b43
> Cr-Commit-Position: refs/heads/master@{#39286}

TBR=jochen@chromium.org,bmeurer@chromium.org,machenbach@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:3782

Review-Url: https://codereview.chromium.org/2328563002
Cr-Commit-Position: refs/heads/master@{#39287}

BUILD.gn

@@ -322,13 +322,6 @@ config("toolchain") {
   if (is_android && v8_android_log_stdout) {
     defines += [ "V8_ANDROID_LOG_STDOUT" ]
   }
-  if (!is_win && !is_clang) {
-    # GCC 6+ can optimize away pointer comparisons to null. This is
-    # problematic as V8 encodes Values through tagged pointers and comparisons
-    # with 0 are actually necessary in many cases. As a temporary Workaround
-    # we disable this optimization. See: https://crbug.com/v8/3782
-    cflags += [ "-fno-delete-null-pointer-checks" ]
-  }
 
   # TODO(jochen): Support v8_enable_prof on Windows.
   # TODO(jochen): Add support for compiling with simulators.

gypfiles/toolchain.gypi

@@ -1098,13 +1098,6 @@
           }],
         ],  # conditions
       }],
-      # GCC 6+ can optimize away pointer comparisons to null. This is
-      # problematic as V8 encodes Values through tagged pointers and comparisons
-      # with 0 are actually necessary in many cases. As a temporary Workaround
-      # we disable this optimization. See: https://crbug.com/v8/3782
-      ['OS!="win" and clang==0', {
-        'cflags': [ '-fno-delete-null-pointer-checks' ]
-      }],
       ['OS=="solaris"', {
         'defines': [ '__C99FEATURES__=1' ],  # isinf() etc.
       }],