[IRSim] Letting call instructions be legal for similarity identification.

Here we let non-intrinsic calls be considered legal and valid for
similarity only if the call is not indirect, and has a name.

For two calls to be considered similar, they must have the same name,
the same function types, and the same set of parameters, including tail
calls and calling conventions.

Tests are found in unittests/Analysis/IRSimilarityIdentifierTest.cpp.

Reviewers: jroelofs, paquette

Differential Revision: https://reviews.llvm.org/D87312