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/x64/lower/isle.rs

@@ -526,9 +526,9 @@ where
     #[inline]
     fn sum_extend_fits_in_32_bits(
         &mut self,
-        offset: Offset32,
         extend_from_ty: Type,
         constant_value: Imm64,
+        offset: Offset32,
     ) -> Option<u32> {
         let offset: i64 = offset.into();
         let constant_value: u64 = constant_value.bits() as u64;

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

@@ -1,4 +1,4 @@
 src/clif.isle 443b34b797fc8ace
 src/prelude.isle a7915a6b88310eb5
-src/isa/x64/inst.isle a63b8ede292f2e20
+src/isa/x64/inst.isle 65f15f51eefe0ce3
 src/isa/x64/lower.isle 4c567e9157f84afb