SPV: For AST interpolateAt* ops consuming a swizzle, invert the order.

Apply the interpolation first, then apply the swizzle to the result,
the inverse of the order requested by the AST.  This fixes issue #411.

Test/spv.swizzleInversion.frag

@@ -0,0 +1,16 @@
+#version 450
+
+in vec4 in4;
+in vec3 in3;
+
+void main()
+{
+    vec3 v43 = interpolateAtCentroid(in4.wzx);
+    vec2 v42 = interpolateAtSample(in4.zx, 1);
+    vec4 v44 = interpolateAtOffset(in4.zyxw, vec2(2.0));
+    float v41 = interpolateAtOffset(in4.y, vec2(2.0));
+
+    vec3 v33 = interpolateAtCentroid(in3.yzx);
+    vec2 v32 = interpolateAtSample(in3.zx, 1);
+    float v31 = interpolateAtOffset(in4.y, vec2(2.0));
+}