freedreno/ir3: Add a NIR pass to select tex instructions eligible for pre-fetch

freedreno/ir3: Add a NIR pass to select tex instructions eligible for pre-fetch

The pass should run once at the end of shader compilation, for a4xx
onwards. It iterates texture sampling instructions and mark those
eligibile for pre-dispatch by changing the tex op from 'tex' to
'tex_prefetch'. An instruction is eligibile if:

* The coordinate is a vector where all its components come from a
  shader input.
* The order of the components match exactly that of the input (no
  swizzles).
* The instruction is in the 'main' function, and in the outer
  most-block.

The first two restrictions were arrived to empirically, so more
testing could tighten or loosen it.

The 3rd restriction is there to allow moving the instructions
eligible for pre-dispatch to the beginning of the shader, so
that we don't block the registers holding the result for too
long.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>