Carsten Haitzler authored Jan 08, 2019 and Emma Anholt committed Jan 28, 2019

Otherwise, the compiler is free to reuse the register containing the input
for another call and assume that the value hasn't been modified.  Fixes
crashes on texture upload/download with current gcc.

We now have to have a temporary for the cpu2 value, since outputs must be
lvalues.

(commit message by anholt)

Fixes: 4d300242 ("vc4: Use NEON to speed up utile loads on Pi2.")