Add a new function attribute 'cold' to functions.

Other than recognizing the attribute, the patch does little else.
It changes the branch probability analyzer so that edges into
blocks postdominated by a cold function are given low weight.

Added analysis and code generation tests.  Added documentation for the
new attribute.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182638 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AsmParser/LLParser.cpp b/lib/AsmParser/LLParser.cpp
index 62d8070..b22d251f 100644
--- a/lib/AsmParser/LLParser.cpp
+++ b/lib/AsmParser/LLParser.cpp
@@ -909,6 +909,7 @@
       continue;
     }
     case lltok::kw_alwaysinline:      B.addAttribute(Attribute::AlwaysInline); break;
+    case lltok::kw_cold:              B.addAttribute(Attribute::Cold); break;
     case lltok::kw_inlinehint:        B.addAttribute(Attribute::InlineHint); break;
     case lltok::kw_minsize:           B.addAttribute(Attribute::MinSize); break;
     case lltok::kw_naked:             B.addAttribute(Attribute::Naked); break;
@@ -1222,6 +1223,7 @@
 
     case lltok::kw_alignstack:
     case lltok::kw_alwaysinline:
+    case lltok::kw_cold:
     case lltok::kw_inlinehint:
     case lltok::kw_minsize:
     case lltok::kw_naked: