x64: Lower extractlane, scalar_to_vector, and splat in ISLE (#4780)

Lower extractlane, scalar_to_vector and splat in ISLE.

This PR also makes some changes to the SinkableLoad api
* change the return type of sink_load to RegMem as there are more functions available for dealing with RegMem
* add reg_mem_to_reg_mem_imm and register it as an automatic conversion

cranelift/codegen/src/machinst/isle.rs

@@ -298,6 +298,24 @@ macro_rules! isle_prelude_methods {
             }
         }
 
+        #[inline]
+        fn ty_32(&mut self, ty: Type) -> Option<Type> {
+            if ty.bits() == 32 {
+                Some(ty)
+            } else {
+                None
+            }
+        }
+
+        #[inline]
+        fn ty_64(&mut self, ty: Type) -> Option<Type> {
+            if ty.bits() == 64 {
+                Some(ty)
+            } else {
+                None
+            }
+        }
+
         #[inline]
         fn ty_32_or_64(&mut self, ty: Type) -> Option<Type> {
             if ty.bits() == 32 || ty.bits() == 64 {