ir_to_mesa: Emit warnings instead of errors for IR that can't be lowered

ir_to_mesa: Emit warnings instead of errors for IR that can't be lowered

Rely on the driver to do the right thing.  This probably means falling
back to software.  Page 88 of the OpenGL 2.1 spec specifically says:

    "A shader should not fail to compile, and a program object should
    not fail to link due to lack of instruction space or lack of
    temporary variables. Implementations should ensure that all valid
    shaders and program objects may be successfully compiled, linked
    and executed."

There is no provision for saying "No" to a valid shader that is
difficult for the hardware to handle, so stop doing that.

On i915 this causes a large number of piglit tests to change from FAIL
to WARN.  The warning is because the driver still emits messages to
stderr like "i915_program_error: Unsupported opcode: BGNLOOP".

It also fixes ES2 conformance CorrectFull_frag and CorrectParse1_frag
on i915 (and probably other hardware that can't handle loops).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>