[Target/ARM] Only enable OptimizeBarrierPass at -O1 and above.

Ideally this is going to be and LLVM IR pass (shared, among others
with AArch64), but for the time being just enable it if consumers
ask us for optimization and not unconditionally.

Discussed with Tim Northover on IRC.

llvm-svn: 237837
diff --git a/llvm/test/CodeGen/ARM/optimize-dmbs-v7.ll b/llvm/test/CodeGen/ARM/optimize-dmbs-v7.ll
index 64f5e20..34a55aa 100644
--- a/llvm/test/CodeGen/ARM/optimize-dmbs-v7.ll
+++ b/llvm/test/CodeGen/ARM/optimize-dmbs-v7.ll
@@ -1,4 +1,4 @@
-; RUN: llc < %s -mtriple=armv7 -mattr=+db | FileCheck %s
+; RUN: llc -O1 < %s -mtriple=armv7 -mattr=+db | FileCheck %s
 
 @x1 = global i32 0, align 4
 @x2 = global i32 0, align 4