Drop -fno-slp-vectorize from the Clang build -- gn edition

The GYP build was updated in
https://crrev.com/5926b3d223f5c183b3341a01b522875aceae2ed9

BUG=none
TBR=brettw

Review URL: https://codereview.chromium.org/585693002

Cr-Commit-Position: refs/heads/master@{#295632}

build/config/compiler/BUILD.gn

@@ -105,14 +105,7 @@ config("compiler") {
     }
   }
 
-  if (is_clang) {
-    cflags += [
-      # TODO(thakis): Remove once http://llvm.org/PR20354 is fixed
-      # and the fix is in chromium.
-      "-fno-slp-vectorize",
-    ]
-
-    if (is_debug) {
+  if (is_clang && is_debug) {
     # Allow comparing the address of references and 'this' against 0
     # in debug builds. Technically, these can never be null in
     # well-defined C/C++ and Clang can optimize such checks away in
@@ -122,7 +115,6 @@ config("compiler") {
       "-Wno-tautological-undefined-compare",
     ]
   }
-  }
 
   if (is_clang && !is_win) {
     # This is here so that all files get recompiled after a clang roll and