Improve code for uniform switches with a 'break' under varying control flow.

Previously, when we had a switch statement with a uniform switch condition
but a 'break' statement that was under varying control flow inside the
switch, we'd promote the switch condition to be varying so that the
break would work correctly.

Now, we leave the condition as uniform and are thus able to use the
more-efficient LLVM switch instruction in this case.

Issue #156.

ctx.h

@@ -580,6 +580,13 @@ private:
         if present), this map gives the basic block for the immediately
         following case/default label. */
     const std::map<llvm::BasicBlock *, llvm::BasicBlock *> *nextBlocks;
+
+    /** Records whether the switch condition was uniform; this is a
+        distinct notion from whether the switch represents uniform or
+        varying control flow; we may have varying control flow from a
+        uniform switch condition if there is a 'break' inside the switch
+        that's under varying control flow. */
+    bool switchConditionWasUniform;
     /** @} */
 
     /** A pointer to memory that records which of the program instances
@@ -634,8 +641,7 @@ private:
 
     void restoreMaskGivenReturns(llvm::Value *oldMask);
     void addSwitchMaskCheck(llvm::Value *mask);
-    bool inUniformSwitch() const;
-    bool inVaryingSwitch() const;
+    bool inSwitchStatement() const;
     llvm::Value *getMaskAtSwitchEntry();
 
     CFInfo *popCFState();