I noticed that a VS I was debugging was missing all of its output stores
-- outputs_written was for POS, VAR0, VAR3, while the shader's variables
were POS, VAR9, and VAR12.  I'm not sure what outputs_written is supposed
to be doing here, but we can just walk the declared variables and avoid
both this bug and the emission of extra stvpms for less-than-vec4
varyings.