[Hexagon] Add SDAG preprocessing step to expose shifted addressing modes

Transform: (store ch addr (add x (add (shl y c) e)))
       to: (store ch addr (add x (shl (add y d) c))),
where e = (shl d c) for some integer d.
The purpose of this is to enable generation of loads/stores with
shifted addressing mode, i.e. mem(x+y<<#c). For that, the shift
value c must be 0, 1 or 2.

llvm-svn: 273466