[Hexagon] [Test] Remove undef and infinite loop from test
The undef and the infinite loop at the end cause this test to be translated
unpredictably. In particular, the checked-for mpy disappears under
certain legal optimizations (e.g. the one in D50222).
Since the use of these constructs is not relevant to the behavior tested,
according to the header comment, this change, suggested by @kparzysz,
Was initially committed in r341046, but was reverted.
Patch by: hermord (Dmytro Shynkevych)!
Reviewed By: kparzysz
Subscribers: lebedev.ri, llvm-commits, kparzysz
Differential Revision: https://reviews.llvm.org/D50944