coverage: Avoid overflow when the MC/DC condition limit is exceeded

If we perform this subtraction first, it can sometimes overflow to -1 before
the addition can bring its value back to 0. That behaviour seems to be benign,
but it nevertheless causes test failures in compiler configurations that check
for overflow.

Author: Zalathar

compiler/rustc_mir_build/src/build/coverageinfo/mcdc.rs

@@ -217,7 +217,7 @@ impl MCDCInfoBuilder {
                 }
                 _ => {
                     // Do not generate mcdc mappings and statements for decisions with too many conditions.
-                    let rebase_idx = self.branch_spans.len() - decision.conditions_num + 1;
+                    let rebase_idx = self.branch_spans.len() + 1 - decision.conditions_num;
                     for branch in &mut self.branch_spans[rebase_idx..] {
                         branch.condition_info = None;
                     }