[machinst x64]: refactor using added RegMem::from(Writable<Reg>)

cranelift/codegen/src/isa/x64/inst/args.rs

@@ -5,7 +5,7 @@ use super::EmitState;
 use crate::ir::condcodes::{FloatCC, IntCC};
 use crate::machinst::*;
 use core::fmt::Debug;
-use regalloc::{RealRegUniverse, Reg, RegClass, RegUsageCollector, RegUsageMapper};
+use regalloc::{RealRegUniverse, Reg, RegClass, RegUsageCollector, RegUsageMapper, Writable};
 use std::fmt;
 use std::string::{String, ToString};
 
@@ -265,6 +265,12 @@ impl RegMem {
     }
 }
 
+impl From<Writable<Reg>> for RegMem {
+    fn from(r: Writable<Reg>) -> Self {
+        RegMem::reg(r.to_reg())
+    }
+}
+
 impl ShowWithRRU for RegMem {
     fn show_rru(&self, mb_rru: Option<&RealRegUniverse>) -> String {
         self.show_rru_sized(mb_rru, 8)