Only save callee-saved registers that are used (#293)

* Only save callee-saved registers that are actually being used.

* Rename AllocatableSet to RegisterSet

* Style cleanup and small renames for readability.

* Adjust x86 prologue-epilogue test to account for callee-saved register optimization.

* Add more tests for prologue-epilogue optimizations.

lib/cretonne/src/isa/arm64/abi.rs

@@ -3,7 +3,7 @@
 use super::registers::{FPR, GPR};
 use ir;
 use isa::RegClass;
-use regalloc::AllocatableSet;
+use regalloc::RegisterSet;
 use settings as shared_settings;
 
 /// Legalize `sig`.
@@ -21,6 +21,6 @@ pub fn regclass_for_abi_type(ty: ir::Type) -> RegClass {
 }
 
 /// Get the set of allocatable registers for `func`.
-pub fn allocatable_registers(_func: &ir::Function) -> AllocatableSet {
+pub fn allocatable_registers(_func: &ir::Function) -> RegisterSet {
     unimplemented!()
 }