[CodeGen] llc silently ignores -{start|stop}-{after|before} options

[optimisan]

Particularly, llc -mtriple=amdgcn-- -start-after=dead-mi-elimination,4 should be running remaining passes in the pipeline after the last instance of dead-mi-elimination, but instead this runs no passes.

[llvmbot]

Hi!

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Akshat Oke (optimisan)

Particularly, `llc -mtriple=amdgcn-- -start-after=dead-mi-elimination,4` should be running remaining passes in the pipeline after the last instance of dead-mi-elimination, but instead this runs no passes.