glsl/lower_precision: Add a unit test that I thought we might fail at.

If you lowered precision too late, it would be easy to break this.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21666>

src/compiler/glsl/tests/lower_precision_test.py

@@ -2287,6 +2287,18 @@ TESTS = [
          """,
          r'\(expression vec4 \* \(swiz xxxx \(var_ref a\) \)\(expression float f162f \(var_ref b\) \) \)'),
 
+    Test("respect copies",
+         """
+         uniform mediump float a, b;
+
+         void main()
+         {
+            highp float x = a;
+            gl_FragColor.x = x * b;
+         }
+         """,
+         r'expression float \* \(expression float f162f \(var_ref a\) \) \(expression float f162f \(var_ref b\) \) '), # should be uint16_t
+
 ]