[ConstraintElim] Bail out on non-dedicated exits when adding exiting conditions (#116627)

This patch bails out non-dedicated exits to avoid adding exiting
conditions to invalid context.
Closes htt

[ConstraintElim] Bail out on non-dedicated exits when adding exiting conditions (#116627)

This patch bails out non-dedicated exits to avoid adding exiting
conditions to invalid context.
Closes https://github.com/llvm/llvm-project/issues/116553.

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[ConstraintElim] Use cond from header as upper bound on IV in exit BB. (#94610)

For loops, we can use the condition in the loop header as upper bound on
the compared induction in the unique exit bl

[ConstraintElim] Use cond from header as upper bound on IV in exit BB. (#94610)

For loops, we can use the condition in the loop header as upper bound on
the compared induction in the unique exit block, if it exists. This can
be done even if there are multiple in-loop edges to the unique exit
block, as any other exit may only exit earlier.

More generally, we could add the OR of all exit conditions to the exit,
but that's a possible future extension.

PR: https://github.com/llvm/llvm-project/pull/94610

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[ConstraintElim] Add multi-exit tests for #94610.

Additional test coverage with multi-exit loops for
https://github.com/llvm/llvm-project/pull/94610.

[ConstraintElim] Add test for mis-compile due to #94610.

Additional test coverage for a miscompile in earlier versions of
https://github.com/llvm/llvm-project/pull/94610.

[ConstraintElim] Add induction tests with different start values.

Extra tests for https://github.com/llvm/llvm-project/pull/94610.

[ConstraintElim] Add set of tests where a loop iv is used in exit.

Test cases inspired by
https://github.com/llvm/llvm-project/issues/90417.