[AArch64][Falkor] Avoid generating STRQro* instructions

Summary:
STRQro* instructions are slower than the alternative ADD/STRQui expanded
instructions on Falkor, so avoid generating them unless we're optimizing
for code size.

Reviewers: t.p.northover, mcrosier

Subscribers: aemerson, rengolin, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D37020

llvm-svn: 311931