This invariant is being violated in the test case
https://reviews.llvm.org/D77849, related to the use of the relatively
new ability for callbr to have return values, and MachineBasicBlocks
with INLINEASM_BR terminators to emit live out register defs.
As noted in the comment, this triggers invariant violations in
MachineVerifier via llc -verify-machineinstrs or
llc -verify-regalloc, since only MachineInstrs that are terminators
are allowed to follow the first terminator.
https://reviews.llvm.org/D75098 may rework this very assertion if we're
spilling via a (proposed) TCOPY MachineInstr.