panfrost/midgard: Implement integer downsize ops

panfrost/midgard: Implement integer downsize ops

Oh, dear. No turning back now.

We begin implementing non-32-bit types, using downsizing integer type
conversions as the initial instructions. We implement them naively as
type-converting moves; substantially more efficient operation is
possible by copypropping the type conversion modifier, but this
optimization is not implemented here.

Size converting modifiers on Midgard allow an instruction to write to a
destination 1/2 the size, or to read from a source 1/2 the size. If we
need an extreme conversion (32-bit to 8-bit, for instance), multiple
type converting ops are chained together, which here is handled via an
algebraic pass.

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