Fixed some error handling for invalid if statements. Started if codegen.

test/if_cond_assign.ink

@@ -0,0 +1,6 @@
+vertex main :: (pos : float3 @position) -> float4 @position {
+	if 0 = 100 {
+		return float4(pos, 1.0);
+	}
+	return float4(0.0);
+}