[enhancement] User-specified `dead_if(predicate)` akin to `assume(predicate)` to enable clock gating

### What's hard to do? (limit 100 words)

Related to #542 -- similar to an "assume(p)" which would conceptually forward-propagate the truth of predicate `p` for subsequent optimization, for clock gating purposes we may want to add something like a "dead_if(p, value)" so you could mark that output values could be clock gated off under certain conditions `p`. Notably this is a semantic change, not a hint. The benefit would be that liveness analysis could reverse-flow to clock gate out all dependent operations under that predicate.

### Current best alternative workaround (limit 100 words)

Right now the value is just always assumed live under all conditions (aside from explicit valid bit specification to XLS causing clock gating).

### Your view of the "best case XLS enhancement" (limit 100 words)

Users can specify dead_if predicates, and ideally it could be orthogonal to the functional block (instead of /having/ to be inline), and that would cause the complete frontier of possible clock gating under that predicate to occur.

No file, test, or entry point is identified. Start by reading related issue #542 and locating XLS's liveness-analysis and clock-gating implementation; completion would require a settled design and tests demonstrating user-specified semantic dead_if predicates propagate to dependent clock gating.

Written by the indexing model from the issue text.