[Hexagon] Use constant extenders to fix up hardware loops

[Hexagon] Use constant extenders to fix up hardware loops

Use a loop instruction with a constant extender for a hardware
loop instruction that is too far away from the start of the loop.
This is cheaper than changing the SA register value.

Differential Revision: http://reviews.llvm.org/D9262

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235882 91177308-0d34-0410-b5e6-96231b3b80d8