ISLE: remove all uses of argument polarity, and remove it from the language. (#4091)

This PR removes "argument polarity": the feature of ISLE extractors that lets them take
inputs aside from the value to be matched.

Cases that need this expressivity have been subsumed by #4072 with if-let clauses;
we can now finally remove this misfeature of the language, which has caused significant
confusion and has always felt like a bit of a hack.

This PR (i) removes the feature from the ISLE compiler; (ii) removes it from the reference
documentation; and (iii) refactors away all uses of the feature in our three existing
backends written in ISLE.

cranelift/codegen/src/isa/s390x/lower/isle.rs

@@ -486,9 +486,9 @@ where
     }
 
     #[inline]
-    fn same_reg(&mut self, src: Reg, dst: WritableReg) -> Option<()> {
+    fn same_reg(&mut self, dst: WritableReg, src: Reg) -> Option<Reg> {
         if dst.to_reg() == src {
-            Some(())
+            Some(src)
         } else {
             None
         }