x64: port load to ISLE (#3993)

This change moves the majority of the lowerings for CLIF's `load`
instruction over to ISLE. To do so, it also migrates the previous
mechanism for creating an `Amode` (`lower_to_amode`) to several ISLE
rules (see `to_amode`).

cranelift/codegen/src/machinst/isle.rs

@@ -245,6 +245,14 @@ macro_rules! isle_prelude_methods {
             }
         }
 
+        #[inline]
+        fn ty_int_bool_ref_64(&mut self, ty: Type) -> Option<Type> {
+            match ty {
+                I64 | B64 | R64 => Some(ty),
+                _ => None,
+            }
+        }
+
         #[inline]
         fn ty_int_bool_128(&mut self, ty: Type) -> Option<Type> {
             match ty {
@@ -441,6 +449,12 @@ macro_rules! isle_prelude_methods {
         fn lane_type(&mut self, ty: Type) -> Type {
             ty.lane_type()
         }
+
+        #[inline]
+        fn offset32_to_u32(&mut self, offset: Offset32) -> u32 {
+            let offset: i32 = offset.into();
+            offset as u32
+        }
     };
 }