vc4: Emit a warning and proceed for handling loops in NIR.

vc4: Emit a warning and proceed for handling loops in NIR.

We don't really suppor control flow yet, but it's a lot nicer to render
something and warn on stderr than to crash.

Fixes the following piglit tests:
- shaders/complex-loop-analysis-bug
- shaders/glsl-fs-discard-04

Converts the following piglit tests from crash to fail:
- shaders/glsl-fs-continue-inside-do-while
- shaders/glsl-fs-loop
- shaders/glsl-fs-loop-continue
- shaders/glsl-fs-loop-nested
- shaders/glsl-texcoord-array
- shaders/glsl-vs-continue-inside-do-while
- shaders/glsl-vs-loop
- shaders/glsl-vs-loop-continue
- shaders/glsl-vs-loop-nested

No piglit regressions.

v2 (Eric): Add stronger stderr warning.

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>