Block pretty printing (partial re-revert of 1177123002).

Revert "Revert of Decompiler improvements. (patchset #2 id:20001 of https://codereview.chromium.org/1177123002/)"

This reverts commit e2ce4681, avoiding the hex->dec changes, and re-introducing the block pretty printing of the original CL.

The hex-dec issue would be handled separately.
BUG=

Review URL: https://codereview.chromium.org/1186273005

Cr-Commit-Position: refs/heads/master@{#29063}

src/compiler/code-generator.cc

@@ -99,12 +99,29 @@ Handle<Code> CodeGenerator::GenerateCode() {
       if (FLAG_code_comments) {
         // TODO(titzer): these code comments are a giant memory leak.
         Vector<char> buffer = Vector<char>::New(200);
-        SNPrintF(buffer, "-- B%d start%s%s%s%s --", block->rpo_number().ToInt(),
+        char* buffer_start = buffer.start();
+
+        int next = SNPrintF(
+            buffer, "-- B%d start%s%s%s%s", block->rpo_number().ToInt(),
             block->IsDeferred() ? " (deferred)" : "",
             block->needs_frame() ? "" : " (no frame)",
             block->must_construct_frame() ? " (construct frame)" : "",
             block->must_deconstruct_frame() ? " (deconstruct frame)" : "");
-        masm()->RecordComment(buffer.start());
+
+        buffer = buffer.SubVector(next, buffer.length());
+
+        if (block->IsLoopHeader()) {
+          next =
+              SNPrintF(buffer, " (loop up to %d)", block->loop_end().ToInt());
+          buffer = buffer.SubVector(next, buffer.length());
+        }
+        if (block->loop_header().IsValid()) {
+          next =
+              SNPrintF(buffer, " (in loop %d)", block->loop_header().ToInt());
+          buffer = buffer.SubVector(next, buffer.length());
+        }
+        SNPrintF(buffer, " --");
+        masm()->RecordComment(buffer_start);
       }
       masm()->bind(GetLabel(current_block_));
       for (int i = block->code_start(); i < block->code_end(); ++i) {