Add initial support for 'goto' statements.

ispc now supports goto, but only under uniform control flow--i.e.
it must be possible for the compiler to statically determine that
all program instances will follow the goto.  An error is issued at
compile time if a goto is used when this is not the case.

tests_errors/goto-3.ispc

@@ -0,0 +1,11 @@
+// "goto" statements are only legal under "uniform" control flow
+
+void func(int x) {
+    cif (x < 0)
+        goto label;
+
+ label:
+        ;
+}
+
+