aarch64: Migrate ishl/ushr/sshr to ISLE (#3608)

* aarch64: Migrate ishl/ushr/sshr to ISLE

This commit migrates the `ishl`, `ushr`, and `sshr` instructions to
ISLE. These involve special cases for almost all types of integers
(including vectors) and helper functions for the i128 lowerings since
the i128 lowerings look to be used for other instructions as well. This
doesn't delete the i128 lowerings in the Rust code just yet because
they're still used by Rust lowerings, but they should be deletable in
due time once those lowerings are translated to ISLE.

* Use more descriptive names for i128 lowerings

* Use a with_flags-lookalike for csel

* Use existing `with_flags_*`

* Coment backwards order

* Update generated code

cranelift/codegen/src/machinst/isle.rs

@@ -105,6 +105,14 @@ macro_rules! isle_prelude_methods {
             ty.bits()
         }
 
+        fn fits_in_16(&mut self, ty: Type) -> Option<Type> {
+            if ty.bits() <= 16 {
+                Some(ty)
+            } else {
+                None
+            }
+        }
+
         #[inline]
         fn fits_in_32(&mut self, ty: Type) -> Option<Type> {
             if ty.bits() <= 32 {