Fix codegen bug with foreach_tiled.

When the outermost dimension(s) were partially active, but the innermost
dimension was all on, we'd inadvertently use an incorrect "all on"
execution mask.

Fixes issues #177 and #200.

stmt.cpp

@@ -1649,11 +1649,12 @@ ForeachStmt::EmitCode(FunctionEmitContext *ctx) const {
     // width.  Set the mask and jump to the masked loop body.
     ctx->SetCurrentBasicBlock(bbAllInnerPartialOuter); {
         llvm::Value *mask;
-        if (extrasMaskPtrs.size() == 0)
+        if (nDims == 1)
             // 1D loop; we shouldn't ever get here anyway
             mask = LLVMMaskAllOff;
         else
-            mask = ctx->LoadInst(extrasMaskPtrs.back());
+            mask = ctx->LoadInst(extrasMaskPtrs[nDims-2]);
+
         ctx->SetInternalMask(mask);
 
         ctx->StoreInst(LLVMTrue, stepIndexAfterMaskedBodyPtr);