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/riscv/mod.rs

@@ -92,7 +92,7 @@ impl TargetIsa for Isa {
         abi::regclass_for_abi_type(ty)
     }
 
-    fn allocatable_registers(&self, func: &ir::Function) -> regalloc::AllocatableSet {
+    fn allocatable_registers(&self, func: &ir::Function) -> regalloc::RegisterSet {
         abi::allocatable_registers(func, &self.isa_flags)
     }