[AliasSet] Fix UnknownInstructions printing

Summary:
AliasSet::print uses `I->printAsOperand` to print UnknownInstructions. The problem is that not all UnknownInstructions have names (e.g. call instructions). When such instructions are printed, they appear as `<badref>` in AliasSets, which is very confusing, as the values are perfectly valid.

This patch fixes that by printing UnknownInstructions without a name using `print` instead of `printAsOperand`.

Reviewers: asbirlea, chandlerc, sanjoy, grosser

Reviewed By: asbirlea

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48609

llvm-svn: 335751