Fix issue with remapping global uniform blocks

Avoid adding global uniform blocks to stages that don't already have it.
Otherwise multiple stages point to the same block object, and a
remapping that occurs later on will change the mapping on multiple
stages.

Test/vk.relaxed.changeSet.frag

@@ -0,0 +1,13 @@
+#version 460
+
+layout(location = 0) out vec4 fragColor;
+
+uniform sampler2D sTexture;
+
+in vec4 Color;
+in vec2 UV;
+
+void main()
+{
+    fragColor = Color * texture(sTexture, UV.st).r;
+}