Implement stack_addr, stack_load, stack_store for x86-64. (#370)

cranelift/filetests/isa/x86/stack-load-store64.cton

@@ -0,0 +1,21 @@
+; legalization of stack load and store instructions on x86-64.
+test legalizer
+set opt_level=fastest
+target x86_64 haswell
+
+function %stack_load_and_store() {
+           ss0 = explicit_slot 8, offset 0
+
+ebb0:
+   v0 = stack_load.i64 ss0
+
+; check: v1 = stack_addr.i64 ss0
+; check: v0 = load.i64 notrap aligned v1
+
+   stack_store.i64 v0, ss0
+
+; check: v2 = stack_addr.i64 ss0
+; check: store notrap aligned v0, v2
+
+   return
+}