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.cpp

@@ -245,6 +245,8 @@ FunctionEmitContext::FunctionEmitContext(Function *func, Symbol *funSym,
     StoreInst(llvm::Constant::getNullValue(LLVMTypes::VoidPointerType), 
               launchGroupHandlePtr);
 
+    disableGSWarningCount = 0;
+
     const Type *returnType = function->GetReturnType();
     if (!returnType || Type::Equal(returnType, AtomicType::Void))
         returnValuePtr = NULL;
@@ -1106,6 +1108,19 @@ FunctionEmitContext::InForeachLoop() const {
 }
 
 
+void
+FunctionEmitContext::DisableGatherScatterWarnings() {
+    ++disableGSWarningCount;
+}
+
+
+void
+FunctionEmitContext::EnableGatherScatterWarnings() {
+    --disableGSWarningCount;
+}
+
+
+
 bool
 FunctionEmitContext::initLabelBBlocks(ASTNode *node, void *data) {
     LabeledStmt *ls = dynamic_cast<LabeledStmt *>(node);
@@ -2492,7 +2507,8 @@ FunctionEmitContext::gather(llvm::Value *ptr, const PointerType *ptrType,
     // Add metadata about the source file location so that the
     // optimization passes can print useful performance warnings if we
     // can't optimize out this gather
-    addGSMetadata(call, currentPos);
+    if (disableGSWarningCount == 0)
+        addGSMetadata(call, currentPos);
 
     return BitCastInst(call, llvmReturnType, "gather_bitcast");
 }
@@ -2801,7 +2817,9 @@ FunctionEmitContext::scatter(llvm::Value *value, llvm::Value *ptr,
     args.push_back(value);
     args.push_back(mask);
     llvm::Value *inst = CallInst(scatterFunc, NULL, args);
-    addGSMetadata(inst, currentPos);
+
+    if (disableGSWarningCount == 0)
+        addGSMetadata(inst, currentPos);
 }