[NFC] Introduce API to detect tokens penetrating LCSSA form

Following discussion in PR56243, we need to somehow detect the situation
when token values penetrate LCSSA form for transforms that require that
it is maintained by all values (for example, to sustain use-def dominance
invarians). This patch introduces a parameter to LCSSA checkers to control
their ignorance about tokens.

Differential Revision: https://reviews.llvm.org/D129983
Reviewed By: efriedma