THIS IS A WORK IN PROGRESS. NOT FOR SUBMISSION.
This pass lowers the is.constant intrinsics into 'true' or 'false'. The API change to instruction simplify needs review and that part should likely also handle select, but this version works well enough to pass the motivating problems. The restart is known to be crude too. This is an extension of the base work in D64963, but I don't want to hijack that one.