MachineInstrBundle: Pass iterators to getBundle(Start|End); NFC

This is a function to go backwards in a block to find the first
instruction in a bundle, so iterator is a more natural choice for
parameter/return rather than a reference to a MachineInstruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285051 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed