ir: better register shrinkage pretty printing

compiler/__main__.py

@@ -77,7 +77,7 @@ def main():
 
         intermediate_representation.pretty_print()
 
-        print(ir.IRRegister.get_registers())
+        ir.IRRegister.pprint()
 
     except CompilationError as e:
         e.location.source = tokens.data

compiler/ir.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import abc
+import collections
 from abc import abstractmethod
 
 from . import nodes
@@ -169,12 +170,17 @@ class IRRegister(IRAssignable):
         return f"%r{self.id}"
 
     @classmethod
-    def get_registers(cls) -> str:
+    def pprint(cls):
         messages = []
+        by_codegen = collections.defaultdict(lambda: [])
         for register in cls.registers.values():
-            messages += [f"%r{register.id} -> {register.codegen()}"]
+            by_codegen[register.codegen()] += [register.id]
 
-        return "\n".join(messages)
+        for remapped_name, registers in by_codegen.items():
+            register_names = (f"%r{register}" for register in registers)
+            messages += [f"Remapped {', '.join(register_names)} to {remapped_name}"]
+
+        print("\n".join(messages))
 
 
 class IRVariable(IRAssignable):