Unexpected full floating precision register when using UNITY_DECLARE_TEX2D_HALF

1. When using UNITY_DECLARE_TEX2D_HALF, there is a full floating precision register in compiled shader. It should be a half floating precision register.

2. 1) Select shader 'Modx-DiffuseOnly-1.shader', and click 'Compile and show code' (Make sure GLES3x checked)
2) You can observe there is a vec4 in the compiled result