[ARM] Swap cmp operands for automatic shifts

Swap the compare operands if the lhs is a shift and the rhs isn't,
as in arm and T2 the shift can be performed by the compare for its
second operand.

Differential Revision: https://reviews.llvm.org/D39004


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316562 91177308-0d34-0410-b5e6-96231b3b80d8