[Hexagon] Remove dead constant assignment in hardware loop pass

[Hexagon] Remove dead constant assignment in hardware loop pass

After converting a loop to a hardware loop, the pass should remove
any unnecessary instructions from the old compare-and-branch
code. This patch removes a dead constant assignment that was
used in the compare instruction.

Differential Revision: http://reviews.llvm.org/D9720

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237373 91177308-0d34-0410-b5e6-96231b3b80d8