Don't issue gather/scatter warnigns in the 'extra' bits of foreach loops.

With AOS data, we can often coalesce the accesses into gathers for the main
part of foreach loops but only fail on the last bits where the mask is not
all on (since the coalescing code doesn't handle mixed masks, yet.) Before,
we'd report success with coalescing and then also report that gathers were needed
for the same accesses that were coalesced, which was a) confusing, and b)
didn't accurately represent what was going on for the majority of the loop
iterations.

ctx.h

@@ -230,6 +230,13 @@ public:
 
     bool InForeachLoop() const;
 
+    /** Temporarily disables emission of performance warnings from gathers
+        and scatters from subsequent code. */
+    void DisableGatherScatterWarnings();
+
+    /** Reenables emission of gather/scatter performance warnings. */
+    void EnableGatherScatterWarnings();
+
     void SetContinueTarget(llvm::BasicBlock *bb) { continueTarget = bb; }
 
     /** Step through the code and find label statements; create a basic
@@ -644,6 +651,10 @@ private:
         tasks launched from the current function. */
     llvm::Value *launchGroupHandlePtr;
 
+    /** Nesting count of the number of times calling code has disabled (and
+        not yet reenabled) gather/scatter performance warnings. */
+    int disableGSWarningCount;
+
     std::map<std::string, llvm::BasicBlock *> labelMap;
 
     static bool initLabelBBlocks(ASTNode *node, void *data);