[JumpThreading][DebugInfo] Propagate DebugLocs when simplifying loads (#157683)

In simplifyPartiallyRedundantLoad we may replace a load with a PHI of
available values in predecessor blocks. As part of this process, we may
need to cast those values, which we do by inserting a new cast at the
end of the predecessor. These cast instructions should take their debug
location from the load instruction, just as the PHI does; we make an
exception if the predecessor does not unconditionally branch to the
load's block, as in that case we are not guaranteed to reach the load
and must therefore drop its debug location.

Found using https://github.com/llvm/llvm-project/pull/107279.

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