Auto merge of #111596 - cjgillot:dominator-bucket, r=Mark-Simulacrum

Process current bucket instead of parent's bucket when starting loop for dominators.

The linked paper by Georgiadis suggests in §2.2.3 to process `bucket[w]` when beginning the loop, instead of `bucket[parent[w]]` when finishing it.

In the test case, we correctly computed `idom[2] = 0` and `sdom[3] = 1`, but the algorithm returned `idom[3] = 1`, instead of the correct value 0, because of the path 0-7-2-3.

This provoked LLVM ICE in rust-lang/rust#111061 (comment). LLVM checks that SSA assignments dominate uses using its own implementation of Lengauer-Tarjan, and saw case where rustc was breaking the dominance property.

r? `@Mark-Simulacrum`

Author: bors