[InlineSpiller] add assert about spills post terminators

Summary:
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.

Reviewers: void, efriedma, arsenm

Reviewed By: efriedma

Subscribers: qcolombet, wdng, hiraditya, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78166