nak/sm50: Various encoding fixes

Most of this is code clean-ups and unifications.  A few things were
missing proper legalization.  Some of it was just plain wrong.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30281>

src/nouveau/compiler/nak/ir.rs

@@ -3314,7 +3314,7 @@ impl_display_for_op!(OpISetP);
 pub struct OpLop2 {
     pub dst: Dst,
 
-    #[src_type(ALU)]
+    #[src_type(B32)]
     pub srcs: [Src; 2],
 
     pub op: LogicOp2,

src/nouveau/compiler/nak/legalize.rs

@@ -169,17 +169,6 @@ pub trait LegalizeBuildHelpers: SSABuilder {
         src.src_ref = val.into();
     }
 
-    fn copy_alu_src_if_cbuf(
-        &mut self,
-        src: &mut Src,
-        reg_file: RegFile,
-        src_type: SrcType,
-    ) {
-        if matches!(src.src_ref, SrcRef::CBuf(_)) {
-            self.copy_alu_src(src, reg_file, src_type);
-        }
-    }
-
     fn copy_alu_src_if_not_reg(
         &mut self,
         src: &mut Src,