panfrost/midgard: Lower source modifiers for ints

panfrost/midgard: Lower source modifiers for ints

On Midgard, float ops support standard source modifiers (abs/neg) and
destination modifiers (sat/pos/round). Integer ops do not support these,
however. To cope, we use native NIR source modifiers for floats, but
lower them away to iabs/ineg for integers, implementing those ops
simultaneously to avoid regressions.

Fixes the integer tests in
dEQP-GLES2.functional.shaders.operator.unary_operator.minus.*

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>