[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
diff --git a/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
index 7557f24..49dc7d6 100644
--- a/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
+++ b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
@@ -52,7 +52,7 @@
                                      cl::Hidden);
 
 static cl::opt<bool> EnableNarrowLdMerge("enable-narrow-ld-merge", cl::Hidden,
-                                         cl::init(true),
+                                         cl::init(false),
                                          cl::desc("Enable narrow load merge"));
 
 namespace llvm {