machinst/x64: teach regalloc what FP instructions are moves;

and cosmetic changes after #1665 landed.

cranelift/codegen/src/machinst/vcode.rs

@@ -137,13 +137,10 @@ impl<I: VCodeInst> VCodeBuilder<I> {
     /// Set the type of a VReg.
     pub fn set_vreg_type(&mut self, vreg: VirtualReg, ty: Type) {
         if self.vcode.vreg_types.len() <= vreg.get_index() {
-            self.vcode.vreg_types.resize(
-                self.vcode.vreg_types.len()
-                    + ((vreg.get_index() + 1) - self.vcode.vreg_types.len()),
-                ir::types::I8,
-            )
+            self.vcode
+                .vreg_types
+                .resize(vreg.get_index() + 1, ir::types::I8);
         }
-
         self.vcode.vreg_types[vreg.get_index()] = ty;
     }