[MIBundles] Move analyzeVirtReg out of MIBundleOperands iterator (NFC).

analyzeVirtReg does not really fit into the iterator and moving it
makes it easier to change the base iterator.

Reviewers: evandro, t.p.northover, paquette, MatzeB, arsenm, qcolombet

Reviewed By: qcolombet

Differential Revision: https://reviews.llvm.org/D70558