Improvements to code generation for "foreach"

Specialize the code for the innermost loop to not do any masking
computations for the innermost dimension for the iterations where
we are certainly working on a full vector's worth of data.

This fix improves performance/code quality of "foreach" such that
it's essentially the same as the equivalent "for" loop.

Fixes issue #151.

ctx.cpp

@@ -576,7 +576,7 @@ FunctionEmitContext::EndLoop() {
 
 
 void
-FunctionEmitContext::StartForeach(llvm::BasicBlock *ct) {
+FunctionEmitContext::StartForeach() {
     // Store the current values of various loop-related state so that we
     // can restore it when we exit this loop.
     llvm::Value *oldMask = GetInternalMask();
@@ -588,7 +588,7 @@ FunctionEmitContext::StartForeach(llvm::BasicBlock *ct) {
 
     continueLanesPtr = AllocaInst(LLVMTypes::MaskType, "foreach_continue_lanes");
     StoreInst(LLVMMaskAllOff, continueLanesPtr);
-    continueTarget = ct;
+    continueTarget = NULL; // should be set by SetContinueTarget()
 
     loopMask = NULL;
 }