Add a very simple cost model to estimate runtime cost of running code.

This is currently only used to decide whether it's worth doing an
"are all lanes running" check at the start of functions--for small
functions, it's not worth the overhead.

The cost is estimated relatively early in compilation (e.g. before
we know if an array access is a scatter/gather or not, before
constant folding, etc.), so there are many known shortcomings.

module.cpp

@@ -708,7 +708,14 @@ lEmitFunctionCode(FunctionEmitContext *ctx, llvm::Function *function,
     // Finally, we can generate code for the function
     if (code != NULL) {
         bool checkMask = (ft->isTask == true) || 
-            (function->hasFnAttr(llvm::Attribute::AlwaysInline) == false);
+            ((function->hasFnAttr(llvm::Attribute::AlwaysInline) == false) &&
+             code->EstimateCost() > 16);
+        // If the body of the function is non-trivial, then we wrap the
+        // entire thing around a varying "cif (true)" test in order to reap
+        // the side-effect benefit of checking to see if the execution mask
+        // is all on and thence having a specialized code path for that
+        // case.  If this is a simple function, then this isn't worth the
+        // code bloat / overhead.
         if (checkMask) {
             bool allTrue[ISPC_MAX_NVEC];
             for (int i = 0; i < g->target.vectorWidth; ++i)