[ConstraintElim] Track and simplify conditions at use.

This patch updates ConstraintElimination to track uses of conditions in
the worklist. This allows simplifying conditions using the context that
holds directly at the condition, instead of where the condition is
defined.

This allows us to catch more cases in practice: there are multiple
code-size changes for CTMark while compile-time remains unchanged:
https://llvm-compile-time-tracker.com/compare.php?from=4b020cca9363bebab4643f89cfa92ab2fcc7976c&to=7a6e84b8f029713c137814cd46eec775d24a54a1&stat=instructions:u

This should help to simplify D151799.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D153660