[builtins] Add Array.p.join fastpath for single element array.

This is resurrecting an optimization from the pre-torque version.
Avoid allocating another sequential string for the result when there's only one
element, just return the ToString-ed element.  This not only saves time writing
to this destination string, but also reduce GC pressure.

The System Health Memory Benchmark (load:media:google_images) exposed this missing
optimization with a 15% regression in memory usage.  Very large external strings
were being copied into V8's heap as sequential string.

Bug: chromium:896612
Change-Id: Ieb61906f64100cdc15bf96f3ebcccb1207f75356
Reviewed-on: https://chromium-review.googlesource.com/c/1316620
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Peter Wong <peter.wm.wong@gmail.com>
Cr-Commit-Position: refs/heads/master@{#57241}

src/builtins/array-join.tq

@@ -222,6 +222,27 @@ module array {
     assert(IsValidPositiveSmi(buffer.totalStringLength));
     if (buffer.totalStringLength == 0) return kEmptyString;
 
+    // Fast path when there's only one buffer element.
+    if (buffer.index == 1) {
+      const fixedArray: FixedArray = buffer.fixedArray;
+      typeswitch (fixedArray[0]) {
+        // When the element is a string, just return it and completely avoid
+        // allocating another string.
+        case (str: String): {
+          return str;
+        }
+
+        // When the element is a smi, use StringRepeat to quickly build a memory
+        // efficient separator repeated string.
+        case (nofSeparators: Number): {
+          return StringRepeat(context, sep, nofSeparators);
+        }
+        case (obj: Object): {
+          unreachable;
+        }
+      }
+    }
+
     const length: uint32 = Convert<uint32>(Unsigned(buffer.totalStringLength));
     const r: String = buffer.isOneByte ? AllocateSeqOneByteString(length) :
                                          AllocateSeqTwoByteString(length);

test/inspector/runtime/console-messages-limits-expected.txt

@@ -4,4 +4,4 @@ Running test: testMaxConsoleMessagesCount
 Messages reported: 1000
 
 Running test: testMaxConsoleMessagesV8Size
-Messages reported: 3
+Messages reported: 5