Make sure always-inline functions get inlined. <rdar://problem/12423986>

Without this change, when the estimated cost for inlining a function with
an "alwaysinline" attribute was lower than the inlining threshold, the
getInlineCost function was returning that estimated cost rather than the
special InlineCost::AlwaysInlineCost value. That is fine in the normal
inlining case, but it can fail when the inliner considers the opportunity
cost of inlining into an internal or linkonce-odr function. It may decide
not to inline the always-inline function in that case. The fix here is just
to make getInlineCost always return the special value for always-inline
functions. I ran into this building clang with libc++. Tablegen failed to
link because of an always-inline function that was not inlined. I have been
unable to reduce the testcase down to a reasonable size.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165367 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/InlineCost.cpp

@@ -142,6 +142,7 @@ class CallAnalyzer : public InstVisitor<CallAnalyzer, bool> {
 
   int getThreshold() { return Threshold; }
   int getCost() { return Cost; }
+  bool isAlwaysInline() { return AlwaysInline; }
 
   // Keep a bunch of stats about the cost savings found so we can print them
   // out when debugging.
@@ -1057,7 +1058,8 @@ InlineCost InlineCostAnalyzer::getInlineCost(CallSite CS, Function *Callee,
   // Check if there was a reason to force inlining or no inlining.
   if (!ShouldInline && CA.getCost() < CA.getThreshold())
     return InlineCost::getNever();
-  if (ShouldInline && CA.getCost() >= CA.getThreshold())
+  if (ShouldInline && (CA.isAlwaysInline() ||
+                       CA.getCost() >= CA.getThreshold()))
     return InlineCost::getAlways();
 
   return llvm::InlineCost::get(CA.getCost(), CA.getThreshold());