[Hexagon][Test] Remove undef and infinite loop from test

Summary:
As suggested in D50222, this has been refactored into a separate patch.

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,
eliminates them.

Patch by: hermord (Dmytro Shynkevych)!

Reviewers: kparzysz

Reviewed By: kparzysz

Subscribers: llvm-commits, kparzysz

Differential Revision: https://reviews.llvm.org/D50944

llvm-svn: 341046