Optimize > and != 0 alpha tests.

There's no need to do rounding in this case.  These are very common.

Also, if the test was !=, convert it to >, which may work better on
PowerVR.
Optimize ComputeFragmentShaderID() a bit.

I've seen it decently high on some profiles (1-2%).  This cuts it down by
12.5% when used in a tight loop.  The temporary was 8%, the ifs were the
other 4.5%.
Optimize ComputeVertexShaderID() a bit.

This is only like 1%.  Along the way found and fixed an overlap of bits.
Merge pull request #5762 from unknownbrackets/gpu-minor

Shader ID optimizations and alpha test optimization