[AMDGPU][Waitcnt] Fix build error: unused variable 'SWaitInst'

https://reviews.llvm.org/rL333556 caused a buildbot failure.

See http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/21876/steps/build_Lld/logs/stdio

/Users/buildslave/as-bldslv9/lld-x86_64-darwin13/llvm.src/lib/Target/AMDGPU/SIInsertWaitcnts.cpp:2007:10: error: unused variable 'SWaitInst' [-Werror,-Wunused-variable]
    auto SWaitInst = BuildMI(EntryBB, EntryBB.getFirstNonPHI(),

The unused variable was for debugging purposes; removing that piece of code
to fix the build.

llvm-svn: 333559