[AArch64] Favor extended reg patterns for sub

Summary:
Favor the extended reg patterns over the shifted reg patterns that match
only the operand shift and not the full sign/zero extend and shift.

Reviewers: jmolloy, t.p.northover

Subscribers: mcrosier, aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D11569

llvm-svn: 243753