Root-cause evidence: ANGLE TranslatorSPIRV output for the same fragment shader
with gl_PointCoord referenced before vs. after gl_FragCoord.

Reserved id kIdFragCoord = 42.
Overview flag kOverviewHasFragCoordMask = 0x80 (decimal 128) is encoded in the
literal of the `OpExtInst %void %NonSemantic.ANGLE 0/128` instruction.

================================================================================
Shader A (BUG) -- gl_PointCoord first, gl_FragCoord second:

    #version 300 es
    precision mediump float;
    out vec4 c;
    void main() {
        vec2 p = gl_PointCoord;
        c = vec4(p, gl_FragCoord.x, 1.0);
    }

Relevant translator output (out/asan/angle_shader_translator -s=e3 -b=v -o):

               OpEntryPoint Fragment %3 "main" %70 %80 %115
               OpDecorate %115 BuiltIn FragCoord
        %115 = OpVariable %26 Input                 ; BuiltIn FragCoord
        %125 = OpExtInst %4 %1 0          <-- overview literal = 0, FragCoord bit NOT set

gl_FragCoord is at GENERIC id %115 (not %42).  Overview marker says "no
FragCoord".  When the dither-emulation transform later runs it will believe the
shader has no FragCoord and add a SECOND one at %42.

================================================================================
Shader B (CORRECT) -- gl_FragCoord first, gl_PointCoord second:

    #version 300 es
    precision mediump float;
    out vec4 c;
    void main() {
        float x = gl_FragCoord.x;
        vec2 p = gl_PointCoord;
        c = vec4(p, x, 1.0);
    }

Relevant translator output:

               OpEntryPoint Fragment %3 "main" %70 %42 %136
               OpDecorate %42 BuiltIn FragCoord
         %42 = OpVariable %26 Input                 ; BuiltIn FragCoord
        %147 = OpExtInst %4 %1 128        <-- overview literal = 0x80, FragCoord bit SET

gl_FragCoord is at RESERVED id %42 = kIdFragCoord.  Overview marker says
"has FragCoord".  Dither transform will reuse %42.

================================================================================
Conclusion: shader-controlled lexical ordering of built-in references desyncs
the overview-flag from the actual presence of BuiltIn FragCoord, because the
mInputVaryings scan in TranslatorSPIRV.cpp:878-921 terminates with `break` at
the gl_PointCoord / gl_SampleID arms instead of `continue`.