[MachineLateInstrsCleanup] Improve compile time for huge functions.

It was discovered that this pass could be slow on huge functions, meaning 20%
compile time instead of the usual ~0.5% (with a test case spending ~19 mins
just in the backend).

The problem related to the necessary clearing of earlier kill flags when a
redundant instruction is removed. With this patch, the handling of kill flags
is now done by maintaining a map instead of scanning backwards in the
function. This remedies the compile time on the huge file fully.

Reviewed By: vpykhtin, arsenm

Differential Revision: https://reviews.llvm.org/D147532

Resolves https://github.com/llvm/llvm-project/issues/61397