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/mirror/class-inl.h b/runtime/mirror/class-inl.h
index c3754d7..b0ff7ea 100644
--- a/runtime/mirror/class-inl.h
+++ b/runtime/mirror/class-inl.h
@@ -649,11 +649,11 @@
 }
 
 inline bool Class::DescriptorEquals(const char* match) {
-  if (UNLIKELY(IsArrayClass())) {
+  if (IsArrayClass()) {
     return match[0] == '[' && GetComponentType()->DescriptorEquals(match + 1);
-  } else if (UNLIKELY(IsPrimitive())) {
+  } else if (IsPrimitive()) {
     return strcmp(Primitive::Descriptor(GetPrimitiveType()), match) == 0;
-  } else if (UNLIKELY(IsProxyClass())) {
+  } else if (IsProxyClass()) {
     return Runtime::Current()->GetClassLinker()->GetDescriptorForProxy(this) == match;
   } else {
     const DexFile& dex_file = GetDexFile();