x64: Migrate the return and fallthrough_return lowerings to ISLE (#4518)

https://github.com/bytecodealliance/wasmtime/pull/4518

cranelift/codegen/src/machinst/isle.rs

@@ -163,6 +163,11 @@ macro_rules! isle_prelude_methods {
             regs.regs()[i]
         }
 
+        #[inline]
+        fn value_regs_len(&mut self, regs: ValueRegs) -> usize {
+            regs.regs().len()
+        }
+
         #[inline]
         fn u8_as_u32(&mut self, x: u8) -> Option<u32> {
             Some(x.into())
@@ -736,6 +741,14 @@ macro_rules! isle_prelude_methods {
             }
         }
 
+        fn range_singleton(&mut self, r: Range) -> Option<usize> {
+            if r.0 + 1 == r.1 {
+                Some(r.0)
+            } else {
+                None
+            }
+        }
+
         fn range_unwrap(&mut self, r: Range) -> Option<(usize, Range)> {
             if r.0 < r.1 {
                 Some((r.0, (r.0 + 1, r.1)))
@@ -752,6 +765,10 @@ macro_rules! isle_prelude_methods {
             regs.only_reg()
         }
 
+        fn writable_regs_get(&mut self, regs: WritableValueRegs, idx: usize) -> WritableReg {
+            regs.regs()[idx]
+        }
+
         fn abi_copy_to_arg_order(&mut self, abi: &ABISig, idx: usize) -> usize {
             abi.copy_to_arg_order(idx)
         }