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.

llvm-svn: 182638
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index d106173..a94c156 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -692,7 +692,8 @@
         I->getKindAsEnum() == Attribute::SanitizeMemory ||
         I->getKindAsEnum() == Attribute::MinSize ||
         I->getKindAsEnum() == Attribute::NoDuplicate ||
-        I->getKindAsEnum() == Attribute::NoBuiltin) {
+        I->getKindAsEnum() == Attribute::NoBuiltin ||
+        I->getKindAsEnum() == Attribute::Cold) {
       if (!isFunction)
           CheckFailed("Attribute '" + I->getKindAsString() +
                       "' only applies to functions!", V);