machinst x64: add Inst::[move|load|store] for choosing the correct x86 instruction

This change primarily adds the ability to lower packed `[move|load|store]` instructions (the vector types were previously unimplemented), but with the addition of the utility `Inst::[move|load|store]` functions it became possible to remove duplicated code (e.g. `stack_load` and `stack_store`) and use these utility functions elsewhere (though not exhaustively).

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

@@ -634,6 +634,16 @@ impl fmt::Display for SseOpcode {
     }
 }
 
+/// This defines the ways a value can be extended: either signed- or zero-extension, or none for
+/// types that are not extended. Contrast with [ExtMode], which defines the widths from and to which
+/// values can be extended.
+#[derive(Clone, PartialEq)]
+pub enum ExtKind {
+    None,
+    SignExtend,
+    ZeroExtend,
+}
+
 /// These indicate ways of extending (widening) a value, using the Intel
 /// naming: B(yte) = u8, W(ord) = u16, L(ong)word = u32, Q(uad)word = u64
 #[derive(Clone, PartialEq)]