AMDGPU: Remove implicit iterator conversions, NFC

Remove remaining implicit conversions from MachineInstrBundleIterator to
MachineInstr* from the AMDGPU backend.  In most cases, I made them less
attractive by preferring MachineInstr& or using a ranged-based for loop.

Once all the backends are fixed I'll make the operator explicit so that
this doesn't bitrot back.

llvm-svn: 274906