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/method_helper-inl.h b/runtime/method_helper-inl.h
index 3a5056a..9af835f 100644
--- a/runtime/method_helper-inl.h
+++ b/runtime/method_helper-inl.h
@@ -26,6 +26,23 @@
 
 namespace art {
 
+inline bool MethodHelper::HasSameNameAndSignature(MethodHelper* other) {
+  const DexFile* dex_file = method_->GetDexFile();
+  const DexFile::MethodId& mid = dex_file->GetMethodId(GetMethod()->GetDexMethodIndex());
+  if (method_->GetDexCache() == other->method_->GetDexCache()) {
+    const DexFile::MethodId& other_mid =
+        dex_file->GetMethodId(other->GetMethod()->GetDexMethodIndex());
+    return mid.name_idx_ == other_mid.name_idx_ && mid.proto_idx_ == other_mid.proto_idx_;
+  }
+  const DexFile* other_dex_file = other->method_->GetDexFile();
+  const DexFile::MethodId& other_mid =
+      other_dex_file->GetMethodId(other->GetMethod()->GetDexMethodIndex());
+  if (!DexFileStringEquals(dex_file, mid.name_idx_, other_dex_file, other_mid.name_idx_)) {
+    return false;  // Name mismatch.
+  }
+  return dex_file->GetMethodSignature(mid) == other_dex_file->GetMethodSignature(other_mid);
+}
+
 inline mirror::Class* MethodHelper::GetClassFromTypeIdx(uint16_t type_idx, bool resolve) {
   mirror::ArtMethod* method = GetMethod();
   mirror::Class* type = method->GetDexCacheResolvedType(type_idx);