fix[codegen]: same symbol jumpdest merge (vyperlang#3982)

Update the assembly optimizer to return `False` when trying to 
optimize two consecutive `JUMPDEST`s with the same symbol. 
It used to return `True` when no change was made (resulting in an
infinite loop)

+ unit tests

tests/unit/compiler/asm/test_asm_optimizer.py

@@ -3,6 +3,7 @@
 from vyper.compiler import compile_code
 from vyper.compiler.phases import CompilerData
 from vyper.compiler.settings import OptimizationLevel, Settings
+from vyper.ir.compile_ir import _merge_jumpdests
 
 codes = [
     """
@@ -123,3 +124,9 @@ def foo():
 
     assert "unused1()" not in asm
     assert "unused2()" not in asm
+
+
+def test_merge_jumpdests():
+    asm = ["_sym_label_0", "JUMP", "PUSH0", "_sym_label_0", "JUMPDEST", "_sym_label_0", "JUMPDEST"]
+
+    assert _merge_jumpdests(asm) is False, "should not return True as no changes were made"

vyper/ir/compile_ir.py

@@ -892,10 +892,11 @@ def _merge_jumpdests(assembly):
                 # replace all instances of _sym_x with _sym_y
                 # (except for _sym_x JUMPDEST - don't want duplicate labels)
                 new_symbol = assembly[i + 2]
-                for j in range(len(assembly)):
-                    if assembly[j] == current_symbol and i != j:
-                        assembly[j] = new_symbol
-                        changed = True
+                if new_symbol != current_symbol:
+                    for j in range(len(assembly)):
+                        if assembly[j] == current_symbol and i != j:
+                            assembly[j] = new_symbol
+                            changed = True
             elif is_symbol(assembly[i + 2]) and assembly[i + 3] == "JUMP":
                 # _sym_x JUMPDEST _sym_y JUMP
                 # replace all instances of _sym_x with _sym_y