[MachineVerifier][GlobalISel] Check that branches have a MBB operand or are...

[MachineVerifier][GlobalISel] Check that branches have a MBB operand or are declared indirect. Add missing properties to G_BRJT, G_BRINDIRECT

Summary:
Teach MachineVerifier to check branches for MBB operands if they are not declared indirect.

Add `isBarrier`, `isIndirectBranch` to `G_BRINDIRECT` and `G_BRJT`.
Without these, `MachineInstr.isConditionalBranch()` was giving a
false-positive for those instructions.

Reviewers: aemerson, qcolombet, dsanders, arsenm

Reviewed By: dsanders

Subscribers: hiraditya, wdng, simoncook, s.egerton, arsenm, rovka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81587