This glsl code:

#version 100

attribute vec3 vertexPosition;
attribute vec2 texPosition;
uniform mat4 projectionMatrix;
varying vec2 texCoord;

void main() {
    gl_Position = projectionMatrix * vec4(vertexPosition, 1.0);
    texCoord = texPosition;
    gl_Position.z = 0.5;
}

Generates this agal:

mov vt0.w, vc0.x
mov vt0.xyz, va0.xyzz
mov v0, vc0
mov v0.xy, va1.xyyy
m44 op, vt0, vc1
mov op, vc0
mov op.z, vc0.y

and these const fields:

"consts": {
"vc0": [
1,
0.5,
0,
0
]
}

The mov op, vc0 instruction (second last) does not seem to make any sense at all. After removing it the shader works as intended.