Improvements to mask management code; removes a number of unnecessary blends.

We now maintain a the distinction between the value of the mask passed into a
function and the "internal" mask within the function that only accounts for
varying control flow within the function.

The full mask (the AND of the function mask and the internal mask) must be used
for assignments to static and global variables, and reference function parameters.
Further, it is the appropriate mask to use for making decisions about varying
control flow.  However, we can use the internal mask for assignments to variables
declared in the current function (including the return value and non-reference
parameters to the function).  Doing so allows us to catch a few more cases where
the internal mask is all on, even if the mask coming into the function wasn't all
on, and thence use moves rather than blends for those assignments.  (Which in
turn can allow additional optimizations to happen.)

Fixes issue #23.

sym.cpp

@@ -52,6 +52,7 @@ Symbol::Symbol(const std::string &n, SourcePos p, const Type *t,
     constValue = NULL;
     storageClass = sc;
     varyingCFDepth = 0;
+    parentFunction = NULL;
 }