Avoid use of std::string where we have const char*.

Removing the ClassHelper caused std::string creation for all calls to
Class::GetDescriptor and a significant performance regression. Make the
std::string an out argument so the caller can maintain it and its life time
while allowing GetDescriptor to return the common const char* case.

Don't generate GC maps when compilation is disabled.

Remove other uses of std::string that are occuring on critical paths.
Use the cheaper SkipClass in CompileMethod in CompilerDriver.
Specialize the utf8 as utf16 comparison code for the common shorter byte
encoding.
Force a bit of inlining, remove some UNLIKELYs (they are prone to pessimizing
code), add some LIKELYs.

x86-64 host 1-thread interpret-only of 57 apks:
Before: 29.539s
After: 23.467s

Regular compile:
Before: 1m35.347s
After: 1m20.056s

Bug: 16853450
Change-Id: Ic705ea24784bee24ab80084d06174cbf87d557ad
diff --git a/runtime/common_throws.cc b/runtime/common_throws.cc
index 970593d..bb48be3 100644
--- a/runtime/common_throws.cc
+++ b/runtime/common_throws.cc
@@ -296,8 +296,9 @@
                            const StringPiece& type, const StringPiece& name)
     SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
   std::ostringstream msg;
+  std::string temp;
   msg << "No " << scope << "field " << name << " of type " << type
-      << " in class " << c->GetDescriptor() << " or its superclasses";
+      << " in class " << c->GetDescriptor(&temp) << " or its superclasses";
   ThrowException(NULL, "Ljava/lang/NoSuchFieldError;", c, msg.str().c_str());
 }
 
@@ -306,8 +307,9 @@
 void ThrowNoSuchMethodError(InvokeType type, mirror::Class* c, const StringPiece& name,
                             const Signature& signature) {
   std::ostringstream msg;
+  std::string temp;
   msg << "No " << type << " method " << name << signature
-      << " in class " << c->GetDescriptor() << " or its super classes";
+      << " in class " << c->GetDescriptor(&temp) << " or its super classes";
   ThrowException(NULL, "Ljava/lang/NoSuchMethodError;", c, msg.str().c_str());
 }