[NPM] Fix bug in llvm/utils/reduce_pipeline.py

Last minute changes in https://reviews.llvm.org/D110908 unfortunately
introduced a bug wrt automatic pipeline expansion. This patch fixes that
as well as gets rid of a few redundant variables.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D113177

GitOrigin-RevId: 7e34d5ead17563a2aa734b8adcc0fcff5373aebb
3 files changed