Use br instead of a conditional when switching on a constant boolean

Author: clubby789

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -329,9 +329,14 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             let llfalse = helper.llbb_with_cleanup(self, targets.otherwise());
             if switch_ty == bx.tcx().types.bool {
                 // Don't generate trivial icmps when switching on bool.
-                match test_value {
-                    0 => bx.cond_br(discr.immediate(), llfalse, lltrue),
-                    1 => bx.cond_br(discr.immediate(), lltrue, llfalse),
+                let discr = discr.immediate();
+                match (bx.const_to_opt_uint(discr), test_value) {
+                    (Some(0), 0) => bx.br(lltrue),
+                    (Some(1), 0) => bx.br(llfalse),
+                    (Some(0), 1) => bx.br(llfalse),
+                    (Some(1), 1) => bx.br(lltrue),
+                    (None, 0) => bx.cond_br(discr, llfalse, lltrue),
+                    (None, 1) => bx.cond_br(discr, lltrue, llfalse),
                     _ => bug!(),
                 }
             } else {