[llvm-exegesis][mips] Add lit test

[mstojanovic]

Summary

Adding a basic lit test for MIPS.

[atanasyan]

LGTM

llvm/test/tools/llvm-exegesis/Mips/latency-by-opcode-name.s

+# RUN: llvm-exegesis -mode=latency -opcode-name=ADD | FileCheck %s
+
+CHECK:      ---
+CHECK-NEXT: mode: latency
+CHECK-NEXT: key:
+CHECK-NEXT:   instructions:
+CHECK-NEXT:     ADD
+CHECK-NEXT: config: ''
+CHECK-NEXT: register_initial_values:
+CHECK-DAG: - '[[REG1:[A-Z0-9]+]]=0x0'
+CHECK-LAST: ...

llvm/test/tools/llvm-exegesis/Mips/lit.local.cfg

+import subprocess
+import lit.util
+
+if not ('Mips' in config.root.targets):
+    # We need support for Mips.
+    config.unsupported = True
+
+elif not ('mips' in config.root.host_triple):
+    # We need to be running on an Mips host.
+    config.unsupported = True
+
+else:
+    # We need libpfm to be installed and allow reading perf counters. We can
+    # only know that at runtime, so we try to measure the latency of an empty
+    # code snippet and bail out on error.
+    llvm_exegesis_exe = lit.util.which('llvm-exegesis', config.llvm_tools_dir)
+    if not llvm_exegesis_exe:
+        print('llvm-exegesis not found')
+        config.unsupported = True
+    else:
+      try:
+          with open(os.devnull, 'w') as quiet:
+              check_llvm_exegesis_result = subprocess.call(
+                [llvm_exegesis_exe, '-mode', 'latency', '-snippets-file', '/dev/null'], stdout=quiet, stderr=quiet)
+      except OSError:
+          print('could not exec llvm-exegesis')
+          config.unsupported = True
+      if not check_llvm_exegesis_result == 0:
+        config.unsupported = True