AMDGPU: Remove implicit ilist iterator conversions, NFC

One of the changes in lib/Target/AMDGPU/AMDGPUMCInstLower.cpp was a new
one.  Previously, bundle iterators and single-instruction iterators
could be compared to each other (comparing on underlying pointers).
I changed a comparison from using `MBB->end()` to using
`MBB->instr_end()`, since both end iterators should point at the some
place anyway.

I don't think the implicit conversion between the two iterator types is
a good idea since it's fairly easy to accidentally compare to the wrong
thing (they aren't always end iterators).  Otherwise I would have just
added the conversion.

Even with that, no there should be functionality change here.

llvm-svn: 250218