Revert "Make compiler/optimizing/ symbols hidden."

This reverts commit e2727154f25e0db9a5bb92af494d8e47b181dfcf.

Reason for revert: Breaks ASAN tests (ODR violation).
Bug: 142365358

Change-Id: I38103d74a1297256c81d90872b6902ff1e9ef7a4
diff --git a/compiler/optimizing/instruction_simplifier_x86.cc b/compiler/optimizing/instruction_simplifier_x86.cc
index 805dd49..2d8f94a 100644
--- a/compiler/optimizing/instruction_simplifier_x86.cc
+++ b/compiler/optimizing/instruction_simplifier_x86.cc
@@ -17,7 +17,7 @@
 #include "instruction_simplifier_x86_shared.h"
 #include "code_generator_x86.h"
 
-namespace art HIDDEN {
+namespace art {
 
 namespace x86 {