Rollup merge of #153659 - oli-obk:brg_dc_merge_prelude, r=petrochenkov

Mark an unreachable match arm as such

Synchronize which elements have code paths and which are unreachable betwen def collector and build reduced graph.

Found while analyzing on how to best merge these two visitors in a reviewable way

r? @petrochenkov

Author: JonathanBrouwer