Upgrade to rustfmt 0.8.0.

Lots of changes this time.

Worked around what looks like a rustfmt bug in parse_inst_operands where
a large match was nested inside Ok().

src/filetest/domtree.rs

@@ -89,7 +89,10 @@ impl SubTest for TestDomtree {
 
         // Now we know that everything in `expected` is consistent with `domtree`.
         // All other EBB's should be either unreachable or the entry block.
-        for ebb in func.layout.ebbs().skip(1).filter(|ebb| !expected.contains_key(&ebb)) {
+        for ebb in func.layout
+                .ebbs()
+                .skip(1)
+                .filter(|ebb| !expected.contains_key(&ebb)) {
             if let Some(got_inst) = domtree.idom(ebb) {
                 return Err(format!("mismatching idoms for renumbered {}:\n\
                                     want: unrechable, got: {}",