[IRSim][IROutliner] Allowing call instructions to be outlined.

We add an extra check to make sure that we do not outline calls to
indirect functions, but outline whatever the IRSimilarityIdentifier
finds with respect to calls.

Tests:
Removing test/Transforms/IROutliner/illegal-calls.ll
Adding test/Transforms/IROutliner/outlining-calls.ll
Adding test/Transforms/IROutliner/illegal-indirect-calls.ll

Excluding DebugInfo this is the last patch for the initial
implementation of the IROutliner!

Reviewers: jroelofs, paquette

Differential Revision: https://reviews.llvm.org/D87314