[NFC] Adding test for clobbering of high registers in Thumb

Prior to the changes from D52010, clobbering Thumb's high registers in
inline asm would cause incorrect code to be generated - or an assertion
failure for debug builds. Now that the issue is no longer reproducible,
this patch adds a MIR test to cover that scenario.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D96335