Rollup merge of #62063 - ecstatic-morse:dataflow-backward-order, r=nagisa
Rollup merge of #62063 - ecstatic-morse:dataflow-backward-order, r=nagisa

Use a more efficient iteration order for backward dataflow

This applies the same basic principle as #62062 to the reverse dataflow analysis used to compute liveness information. It is functionally equivalent, except that post-order is used instead of reverse post-order.

In the long-term, `BitDenotation` should probably be extended to support both forward and backward dataflow, but there's some more work needed to get to that point.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust