Predicated 'if' statement performance improvements.

Go back to running both sides of 'if' statements with masking and without
branching if we can determine that the code is relatively simple (as per
the simple cost model), and is safe to run even if the mask is 'all off'.
This gives a bit of a performance improvement for some of the examples
(most notably, the ray tracer), and is the code that one wants generated
in this case anyhow.

module.cpp

@@ -705,7 +705,7 @@ lEmitFunctionCode(FunctionEmitContext *ctx, llvm::Function *function,
         int costEstimate = code->EstimateCost();
         bool checkMask = (ft->isTask == true) || 
             ((function->hasFnAttr(llvm::Attribute::AlwaysInline) == false) &&
-             costEstimate > 16);
+             costEstimate > CHECK_MASK_AT_FUNCTION_START_COST);
         Debug(code->pos, "Estimated cost for function \"%s\" = %d\n", 
               funSym->name.c_str(), costEstimate);
         // If the body of the function is non-trivial, then we wrap the