[ARM] Use correct TRAP opcode for thumb in FastISel

We were previously unconditionally using the ARM::TRAP opcode, even
under Thumb. My understanding is that these are essentially the same
thing (they both result in a trap under Thumb), but the ARM::TRAP opcode
is marked as requiring IsARM, so it is more correct to use ARM::tTRAP.

Differential Revision: https://reviews.llvm.org/D72075