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/goto-3.ispc

@@ -0,0 +1,18 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    float b = 0.; b = a; 
+    RET[programIndex] = a+b; 
+    if (all(a == 0))
+        goto skip;
+    RET[programIndex] = 0; 
+ skip:
+    ;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}