Fix #335: Introduce variable size recipes and remove GPR_SAFE reg classes (#552)

* Rename size to base_size and introduce a compute_size function;

* Add infra to inspect in/outs registers when computing the size of an instruction;

* Remove the GPR_SAFE_DEREF and GPR_ZERO_DEREF_SAFE register classes on x86 (fixes #335);

lib/codegen/src/ir/stackslot.rs

@@ -63,7 +63,7 @@ pub enum StackSlotKind {
     /// An emergency spill slot.
     ///
     /// Emergency slots are allocated late when the register's constraint solver needs extra space
-    /// to shuffle registers around. The are only used briefly, and can be reused.
+    /// to shuffle registers around. They are only used briefly, and can be reused.
     EmergencySlot,
 }