This is another instance of the logic from #183159. If we know
one source is not-infinity, and the other source is less than or
equal to 1, this cannot overflow. Special case llvm.amdgcn.trig.preop,
as a substitute for proper range tracking. This almost enables pruning
edge case handling in trig function implementations, if not for the
recursion depth limit (but that's a problem for another day).
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