[Hexagon] Aid bit-reverse load intrinsics lowering with bitcode

The conversion of operatios to bitcode helps to eliminate an additional
store in certain cases. We used to lower these load intrinsics in DAG to
DAG conversion by which time, the "Dead Store Elimination" pass is
already run. There is an associated LLVM patch.
    
Patch by Sumanth Gundapaneni.

llvm-svn: 328776