Steps to reproduce:
1. Open attached "Case 791476 repro" project
2. Select "test.shader" shader and click "Compile and show code"
3. In compiled shader on line 410 notice that instead of comparing with tb4.y, it compares with tb4.x again.

Work arounds:
a. In "test.shader" change line 53 to "float2 thresholded = (inp < 0.2) ? 0.0f : 0.5f;".
b. In "test.shader" on line 65 set the 1st argument of PixelFilteringMean to col.rg instead of "i.uv".

Reproduced with: Unity 5.4.0b15; 5.3.4p4; 5.3.4f1; 5.1.4f1;