[llvm] [update_mir_test_checks] Add missing MIFlags (PR #150012)

[David Green via llvm-commits]

davemgreen wrote:

Arm uses 't' for Thumb instructions and 't2' for Thumb2 instructions I'm afraid.  I'm not sure why that was used but it helps distinguish them from the normal "Arm" mode instructions, like `tADDi8` vs `t2ADDri` vs `ADDri`. If those were accounted for then maybe it would work? But this sounds OK to me either way.

https://github.com/llvm/llvm-project/pull/150012