Move the nop lowering to ISLE, and remove the final return from lower.rs (#4809)

Lower nop in ISLE in the x64 backend, and remove the final Ok(()) from the lower function to assert that all cases that aren't handled in ISLE will panic.

cranelift/codegen/src/isa/x64/lower.isle

@@ -3758,3 +3758,9 @@
 
         ;; SHUFPS xmm_y, xmm_xmp, 0x88
         (x64_shufps dst zeros 0x88)))
+
+;; Rules for `nop` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (nop))
+      (invalid_reg))
+

cranelift/codegen/src/isa/x64/lower.rs

@@ -459,7 +459,8 @@ fn lower_insn_to_regs(
         | Opcode::Isplit
         | Opcode::TlsValue
         | Opcode::SqmulRoundSat
-        | Opcode::Uunarrow => {
+        | Opcode::Uunarrow
+        | Opcode::Nop => {
             let ty = if outputs.len() > 0 {
                 Some(ctx.output_ty(insn, 0))
             } else {
@@ -572,15 +573,9 @@ fn lower_insn_to_regs(
         | Opcode::BrTable => {
             panic!("Branch opcode reached non-branch lowering logic!");
         }
-
-        Opcode::Nop => {
-            // Nothing.
     }
 }
 
-    Ok(())
-}
-
 //=============================================================================
 // Lowering-backend trait implementation.