[ARM] Deliberately prevent inline asm in low overhead loops. NFC

This was already something that was handled by one of the "else"
branches in maybeLoweredToCall, so this patch is an NFC but makes it
explicit and adds a test. We may in the future want to support this
under certain situations but for the moment just don't try and create
low overhead loops with inline asm in them.

Differential Revision: https://reviews.llvm.org/D91257