[AArch64] Disable narrow load merge by default

Summary:
As this optimization converts two loads into one load with two shift instructions,
it could potentially hurt performance if a loop is arithmetic operation intensive.

Reviewers: t.p.northover, mcrosier, jmolloy

Subscribers: evandro, jmolloy, aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D20172

llvm-svn: 270251