[Mips][llvm-exegesis] Add a Mips target

The target does just enough to be able to run llvm-exegesis in latency
mode for at least some opcodes.

Patch by Miloš Stojanović.

Differential Revision: https://reviews.llvm.org/D68649

llvm-svn: 374590
diff --git a/llvm/tools/llvm-exegesis/lib/Assembler.cpp b/llvm/tools/llvm-exegesis/lib/Assembler.cpp
index 4067098..c2b304f 100644
--- a/llvm/tools/llvm-exegesis/lib/Assembler.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Assembler.cpp
@@ -227,9 +227,11 @@
   ET.addTargetSpecificPasses(PM);
   TPC->printAndVerify("After ExegesisTarget::addTargetSpecificPasses");
   // Adding the following passes:
+  // - postrapseudos: expands pseudo return instructions used on some targets.
   // - machineverifier: checks that the MachineFunction is well formed.
   // - prologepilog: saves and restore callee saved registers.
-  for (const char *PassName : {"machineverifier", "prologepilog"})
+  for (const char *PassName :
+       {"postrapseudos", "machineverifier", "prologepilog"})
     if (addPass(PM, PassName, *TPC))
       report_fatal_error("Unable to add a mandatory pass");
   TPC->setInitialized();