x64: fix assert when calling a function

lib/compiler-singlepass/src/machine_x64.rs

@@ -1807,16 +1807,22 @@ impl Machine for MachineX86_64 {
     fn push_location_for_native(&mut self, loc: Location) {
         match loc {
             Location::Imm64(_) => {
-                // Push R9 value slot to be exchange with `mov`.
+                // x86_64 does not support `mov imm64, mem`. We must first place the immdiate value
+                // into a register and then write the register to the memory. Now the problem is
+                // that there might not be any registers available to clobber. In order to make
+                // this work out we spill a register thus retaining both the original value of the
+                // register and producing the required data at the top of the stack.
+                //
+                // FIXME(#2723): figure out how to not require spilling a register here. It should
+                // definitely be possible to `pick_gpr`/`pick_temp_gpr` to grab an otherwise unused
+                // register and just clobber its value here.
                 self.assembler.emit_push(Size::S64, Location::GPR(GPR::R9));
-                self.reserve_unused_temp_gpr(GPR::R9);
                 self.move_location(Size::S64, loc, Location::GPR(GPR::R9));
                 self.assembler.emit_xchg(
                     Size::S64,
                     Location::GPR(GPR::R9),
                     Location::Memory(GPR::RSP, 0),
                 );
-                self.release_gpr(GPR::R9);
             }
             Location::SIMD(_) => {
                 // Dummy value slot to be filled with `mov`.