[regexp] Fix incorrect DCHECK in FixSingleCharacterDisjunctions

The condition only applies in unicode mode, where any lone surrogates
are desugared into a character class (and will not be considered in this
optimization). Non-unicode mode treats lone surrogates exactly like
any other codepoint.

BUG=chromium:711092

Review-Url: https://codereview.chromium.org/2808403006
Cr-Commit-Position: refs/heads/master@{#44638}

src/regexp/jsregexp.cc

@@ -5352,6 +5352,7 @@ void RegExpDisjunction::FixSingleCharacterDisjunctions(
   Zone* zone = compiler->zone();
   ZoneList<RegExpTree*>* alternatives = this->alternatives();
   int length = alternatives->length();
+  const bool unicode = compiler->unicode();
 
   int write_posn = 0;
   int i = 0;
@@ -5368,7 +5369,8 @@ void RegExpDisjunction::FixSingleCharacterDisjunctions(
       i++;
       continue;
     }
-    DCHECK(!unibrow::Utf16::IsLeadSurrogate(atom->data().at(0)));
+    DCHECK_IMPLIES(unicode,
+                   !unibrow::Utf16::IsLeadSurrogate(atom->data().at(0)));
     bool contains_trail_surrogate =
         unibrow::Utf16::IsTrailSurrogate(atom->data().at(0));
     int first_in_run = i;
@@ -5378,7 +5380,8 @@ void RegExpDisjunction::FixSingleCharacterDisjunctions(
       if (!alternative->IsAtom()) break;
       atom = alternative->AsAtom();
       if (atom->length() != 1) break;
-      DCHECK(!unibrow::Utf16::IsLeadSurrogate(atom->data().at(0)));
+      DCHECK_IMPLIES(unicode,
+                     !unibrow::Utf16::IsLeadSurrogate(atom->data().at(0)));
       contains_trail_surrogate |=
           unibrow::Utf16::IsTrailSurrogate(atom->data().at(0));
       i++;
@@ -5394,7 +5397,7 @@ void RegExpDisjunction::FixSingleCharacterDisjunctions(
         ranges->Add(CharacterRange::Singleton(old_atom->data().at(0)), zone);
       }
       RegExpCharacterClass::Flags flags;
-      if (compiler->unicode() && contains_trail_surrogate) {
+      if (unicode && contains_trail_surrogate) {
         flags = RegExpCharacterClass::CONTAINS_SPLIT_SURROGATE;
       }
       alternatives->at(write_posn++) =

test/mjsunit/regexp.js

@@ -803,3 +803,8 @@ assertTrue(/^[\444]*$/.test("\u{24}4"));
 assertTrue(/^[\d-X]*$/.test("234-X-432"));  // CharacterRangeOrUnion.
 assertTrue(/^[\d-X-Z]*$/.test("234-XZ-432"));
 assertFalse(/^[\d-X-Z]*$/.test("234-XYZ-432"));
+
+// Lone leading surrogates. Just here to exercise specific parsing code-paths.
+
+assertFalse(/\uDB88|\uDBEC|aa/.test(""));
+assertFalse(/\uDB88|\uDBEC|aa/u.test(""));