ART: Add Mips32r6 backend support

Add Mips32r6 compiler support.

Don't use deprecated Mips32r2 instructions if running in Mips32r6
mode.

Change-Id: I54e689aa8c026ccb75c4af515aa2794f471c9f67
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 67c96fd..44b07e2 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -877,15 +877,7 @@
     }
 
     if (compiler_filter_string == nullptr) {
-      if (instruction_set_ == kMips &&
-          reinterpret_cast<const MipsInstructionSetFeatures*>(instruction_set_features_.get())->
-          IsR6()) {
-        // For R6, only interpreter mode is working.
-        // TODO: fix compiler for Mips32r6.
-        compiler_filter_string = "interpret-only";
-      } else {
-        compiler_filter_string = "speed";
-      }
+      compiler_filter_string = "speed";
     }
 
     CHECK(compiler_filter_string != nullptr);