[AArch64] Port SIMD narrowing to ISLE (#4478)

* [AArch64] Port SIMD narrowing to ISLE

Fvdemote, snarrow, unarrow and uunarrow.

Also refactor the aarch64 instructions descriptions to parameterize
on ScalarSize instead of using different opcodes.

The zero_value pure constructor has been introduced and used by the
integer narrow operations and it replaces, and extends, the compare
zero patterns.

Copright (c) 2022, Arm Limited.

* use short 'if' patterns

cranelift/codegen/src/isa/aarch64/lower/isle.rs

@@ -424,25 +424,4 @@ where
             _ => panic!(),
         }
     }
-
-    fn zero_value(&mut self, value: Imm64) -> Option<Imm64> {
-        if value.bits() == 0 {
-            return Some(value);
-        }
-        None
-    }
-
-    fn zero_value_f32(&mut self, value: Ieee32) -> Option<Ieee32> {
-        if value.bits() == 0 {
-            return Some(value);
-        }
-        None
-    }
-
-    fn zero_value_f64(&mut self, value: Ieee64) -> Option<Ieee64> {
-        if value.bits() == 0 {
-            return Some(value);
-        }
-        None
-    }
 }