Revert "llvm/lib/CodeGen/TargetSchedule.cpp:132:12: warning: Assert statement...

Revert "llvm/lib/CodeGen/TargetSchedule.cpp:132:12: warning: Assert statement modifies 'NIter'" (#91079)

Reverts llvm/llvm-project#90982

NIter was only declared in !NDEBUG, and only used for assertions - so it
was correct that it was incremented inside the assertion. (& in fact now
the non-asserts build fails, because the variable is incremented even
though it isn't declared)