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

Bots are unhappy:

/Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm/test/CodeGen/Hexagon/swp-const-tc2.ll:10:14: error: CHECK-NOT: excluded string found in input
; CHECK-NOT: = mpy
             ^
<stdin>:22:6: note: found here
 r5 += mpyi(r2,r3)
     ^~~~~

This reverts commit r341046.

llvm-svn: 341049