Change scope of temporaries in match guards
Each pattern in a match arm has its own copy of the match guard in MIR, with its own temporary, so it has to be dropped before the the guards are joined to the single copy of the arm. This PR changes `then_else_break` to allow it to put the temporary in the innermost scope possible. This change isn't done for `if` expressions because that affects a large number of mir-opt tests and could more significantly affect performance.
closes#88649
r? `@oli-obk`
1c858ba5bf