[M68k] fix test regression introduced by D140180

I added a new pass, callbrprepare, to the pass pipelines in
commit a3a84c9 ("[llvm] add CallBrPrepare pass to pipelines")
but did not test experimental backends.

llvm/test/CodeGen/M68k/pipeline.ll

@@ -41,6 +41,7 @@
 ; CHECK-NEXT:      CodeGen Prepare
 ; CHECK-NEXT:      Dominator Tree Construction
 ; CHECK-NEXT:      Exception handling preparation
+; CHECK-NEXT:      Prepare callbr
 ; CHECK-NEXT:      Safe Stack instrumentation pass
 ; CHECK-NEXT:      Insert stack protectors
 ; CHECK-NEXT:      Module Verifier