[regexp] Fold repeated assertions

For instance, /\b\b\b\B\B\B/ is folded into the equivalent /\b\B/.

BUG=v8:6126

Review-Url: https://codereview.chromium.org/2768443003
Cr-Commit-Position: refs/heads/master@{#44004}

src/regexp/regexp-parser.cc

@@ -1691,6 +1691,12 @@ void RegExpBuilder::AddTerm(RegExpTree* term) {
 
 void RegExpBuilder::AddAssertion(RegExpTree* assert) {
   FlushText();
+  if (terms_.length() > 0 && terms_.last()->IsAssertion()) {
+    // Omit repeated assertions of the same type.
+    RegExpAssertion* last = terms_.last()->AsAssertion();
+    RegExpAssertion* next = assert->AsAssertion();
+    if (last->assertion_type() == next->assertion_type()) return;
+  }
   terms_.Add(assert, zone());
   LAST(ADD_ASSERT);
 }

test/cctest/test-regexp.cc

@@ -314,6 +314,9 @@ void TestRegExpParser(bool lookbehind) {
   CheckParseEq("\\u0034", "'\x34'");
   CheckParseEq("\\u003z", "'u003z'");
   CheckParseEq("foo[z]*", "(: 'foo' (# 0 - g [z]))");
+  CheckParseEq("^^^$$$\\b\\b\\b\\b", "(: @^i @$i @b)");
+  CheckParseEq("\\b\\b\\b\\b\\B\\B\\B\\B\\b\\b\\b\\b", "(: @b @B @b)");
+  CheckParseEq("\\b\\B\\b", "(: @b @B @b)");
 
   // Unicode regexps
   CheckParseEq("\\u{12345}", "'\\ud808\\udf45'", true);