Fix missing nargs setting in abioverride test (#57144)

The only place that really reads this is the debug info generation,
which is not enabled by default, but this test would crash if you ran it
on a debug build of julia.

Compiler/test/abioverride.jl

@@ -40,6 +40,7 @@ let world = Base.tls_world_age()
     ## Remove the argument
     resize!(new_source.slotnames, 2)
     resize!(new_source.slotflags, 2)
+    new_source.nargs = 2
 
     # Construct the CodeInstance from the modified CodeInfo data
     global new_ci = Core.CodeInstance(Core.ABIOverride(Tuple{typeof(myplus), Int}, mi),