Unlike LLVM IR `Instruction::eraseFromParent()`,
`MachineInstr::eraseFromParent()` is void and does not return the
iterator following the erased instruction. Returning an iterator can be
very helpful for example when we are erasing MachineInstrs while
iterating, as it provides a convenient way to get a valid iterator.
This patch updates `MachineInstr::eraseFromParent()` to return a
`MachineBlock::iterator` (which is a
`MachineInstrBundleIterator<MachineInstr>`). If the erased instruction
is the head of a bundle, then the returned iterator points to the next
bundle (see unittest).
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