[AArch64] Add new subtarget feature to fold LSL into address mode.

Summary:
This feature enables folding of logical shift operations of up to 3 places into addressing mode on Kryo and Falkor that have a fastpath LSL.

Reviewers: mcrosier, rengolin, t.p.northover

Subscribers: junbuml, gberry, llvm-commits, aemerson

Differential Revision: https://reviews.llvm.org/D31113

llvm-svn: 299240