Auto merge of #139292 - compiler-errors:folder-experiment-7, r=lqd

Folder experiment: Micro-optimize RegionEraserVisitor

**NOTE:** This is one of a series of perf experiments that I've come up with while sick in bed. I'm assigning them to lqd b/c you're a good reviewer and you'll hopefully be awake when these experiments finish, lol.

r? lqd

The region eraser is very hot, so let's see if we can avoid erasing types (and visiting consts and preds that don't have region-ful types) unnecessarily.

Author: bors