IR: Fix use-list-order round-tripping for br

Fix the use-list-order for br instructions by setting the operands in
order of their index to match the use-list-order prediction. The case
where this matters is when there is a condition but the if-true and
if-false branches are identical.

Bug was found when reviewing failures pointed at by
https://reviews.llvm.org/D104950. Fix is similar to
3cf415c6.

Differential Revision: https://reviews.llvm.org/D104959