freedreno/afuc: Handle xmov modifiers

Although it's kind-of similar to "(rptN)" in the shader ISA, I called it
"xmov" to make it clear that it's completely orthogonal to "(rep)",
although you certainly can use both modifiers on the same instruction.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6368>