Fix type checker not promoting ints. Fix some issues in the codegen.

test/simple_float2_expressions.shd

@@ -0,0 +1,14 @@
+vertex main :: (pos : float4 @position) -> float4 @position {
+	src_p0 : float2 = float2(0.0, 1.0);
+	src_p1 : float2 = float2(1.0, 0.0);
+
+	src_half_size : float2 = (src_p1 - src_p0) / 2;
+	src_center    : float2 = (src_p1 + src_p0) / 2;
+	src_pos       : float2 = float2(pos.x, pos.y) * src_half_size + src_center;
+
+	return float4(1, 1, 1, 1);
+}
+
+pixel main :: () -> float4 @target0 {
+	return float4(1, 1, 1, 1);
+}