[MachineVerifier] Doing ::calcRegsPassed in RPO: ~35% faster MV, NFC

Depending on the target, test suite, pipeline config and perhaps other
factors machine verifier when forced on with -verify-machineinstrs can
increase compile time 2-2.5 times over (Release, Asserts On), taking up
~60% of the time. An invaluable tool, it significantly slows down
machine verifier-enabled testing.

Nearly 75% of its time MachineVerifier spends in the calcRegsPassed
method. It's a classic forward dataflow analysis executed over sets, but
visiting MBBs in arbitrary order. We switch that to RPO here.

This speeds up MachineVerifier by about 35%, decreasing the overall
compile time with -verify-machineinstrs by 20-25% or so.

calcRegsPassed itself gets 2x faster here.

All measured on a large suite of shaders targeting a number of GPUs.

Reviewers: bogner, stoklund, rudkx, qcolombet

Reviewed By: bogner

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75032