Steps to reproduce:

1. Download and import attached project. (repro-case_738205-PerformanceTest.zip)
2. Select Shader "Mobile Test".
3. Press "Compile and show code".
4. Look at 45'th line of shader code. (may differ, or search for line " uniform mediump sampler2D _MainTex; ", shown in a screenshot)

Expected result: There should not be multiple conversions between highp -> lowp.
Actual result: unity_Lightmap is always sampled as lowp and then converted back to highp.

Reproduced on: 5.3.0f4, 5.3.0b3, 5.2.2f1, 5.1.4f1