[ARM] Prefer lsls+lsrs over lsls+ands or lsrs+ands in Thumb1.

Saves materializing the immediate for the "ands".

Corresponding patterns exist for lsrs+lsls, but that seems less common
in practice.

Now implemented as a DAGCombine.

Differential Revision: https://reviews.llvm.org/D49585

llvm-svn: 337945