[Local] Treat calls that may not return as being alive.

With the addition of the `willreturn` attribute, functions that may
not return (e.g. due to an infinite loop) are well defined, if they are
not marked as `willreturn`.

This patch updates `wouldInstructionBeTriviallyDead` to not consider
calls that may not return as dead.

This patch still provides an escape hatch for intrinsics, which are
still assumed as willreturn unconditionally. It will be removed once
all intrinsics definitions have been reviewed and updated.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D94106