[GlobalISel] Combiner: Observer-based DCE and retrying of combines

Continues the work for disabling fixed-point iteration in the Combiner
(#94291).

This introduces improved Observer-based heuristics in the
GISel Combiner to retry combining defs/uses of modified instructions and
for performing sparse dead code elimination.

I have experimented a lot with the heuristics and this seems to be the
minimal set of heuristics that allows disabling fixed-point iteration
for AArch64 CTMark O2 without regressions.
Enabling this globally would pass all regression tests for all official
targets (apart from small benign diffs), but I have made this fully
opt-in for now, because I can't quantify the impact for other targets.

This should mostly be on-par with how the WorkList-aware functions
in the InstCombiner and DAGCombiner handle rescheduling instructions
for recombining.

For performance numbers see my follow-up patch for AArch64 (#102167)

Pull Request: https://github.com/llvm/llvm-project/pull/102163

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