Be less aggressive about hinting in RAFast.

In particular, don't spill dirty registers only to satisfy a hint. It is
not worth it.

The attached test case provides an example where the fast allocator
would spill a register when other registers are available.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132900 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/RegAllocFast.cpp

@@ -487,14 +487,12 @@ void RAFast::allocVirtReg(MachineInstr *MI, LiveRegEntry &LRE, unsigned Hint) {
 
   // Take hint when possible.
   if (Hint) {
-    switch(calcSpillCost(Hint)) {
-    default:
-      definePhysReg(MI, Hint, regFree);
-      // Fall through.
-    case 0:
+    // Ignore the hint if we would have to spill a dirty register.
+    unsigned Cost = calcSpillCost(Hint);
+    if (Cost < spillDirty) {
+      if (Cost)
+        definePhysReg(MI, Hint, regFree);
       return assignVirtToPhysReg(LRE, Hint);
-    case spillImpossible:
-      break;
     }
   }