region_infer: BitMatrix representation of region values

This should be more efficient than allocating two BTreeSets for every
region variable?—as it is written in rust-lang#45670.

Author: zackmdavis

Diff for: src/librustc_mir/borrow_check/nll/mod.rs

@@ -160,7 +160,7 @@ fn dump_mir_results<'a, 'gcx, 'tcx>(
         match pass_where {
             // Before the CFG, dump out the values for each region variable.
             PassWhere::BeforeCFG => for region in regioncx.regions() {
-                writeln!(out, "| {:?}: {:?}", region, regioncx.region_value(region))?;
+                writeln!(out, "| {:?}: {}", region, regioncx.region_value_str(region))?;
             },
 
             // Before each basic block, dump out the values