[TrivialDeadness] Introduce API separating two different usages

The earlier usage of wouldInstructionBeTriviallyDead is based on the
assumption that the use_count of that instruction being checked will be
zero. This patch separates the API into two different ones:

1. The strictly conservative one where the instruction is trivially dead iff the uses are dead.
2. The slightly relaxed form, where an instruction is dead along paths where it is not used.

The second form can be used in identifying instructions that are valid
to sink down to uses (D109917).

Reviewed-By: reames
Differential Revision: https://reviews.llvm.org/D114647