[llc] Use -filetype=null to disable MIR printing

If you use -stop-after or similar options, llc will normally print MIR.
This patch checks for -filetype=null as a special case to disable MIR
printing. As the comment says, "The Null output is intended for use for
performance analysis ...", and I found this useful for timing a subset
of the passes that llc runs without the significant overhead of printing
MIR just to send it to /dev/null.

Differential Revision: https://reviews.llvm.org/D89476

llvm/lib/CodeGen/LLVMTargetMachine.cpp

@@ -196,11 +196,14 @@ bool LLVMTargetMachine::addPassesToEmitFile(
   if (!PassConfig)
     return true;
 
-  if (!TargetPassConfig::willCompleteCodeGenPipeline())
-    PM.add(createPrintMIRPass(Out));
-  else if (addAsmPrinter(PM, Out, DwoOut, FileType,
-                           MMIWP->getMMI().getContext()))
-    return true;
+  if (TargetPassConfig::willCompleteCodeGenPipeline()) {
+    if (addAsmPrinter(PM, Out, DwoOut, FileType, MMIWP->getMMI().getContext()))
+      return true;
+  } else {
+    // MIR printing is redundant with -filetype=null.
+    if (FileType != CGFT_Null)
+      PM.add(createPrintMIRPass(Out));
+  }
 
   PM.add(createFreeMachineFunctionPass());
   return false;

llvm/test/tools/llc/filetype-null-stop-after.ll

@@ -0,0 +1,3 @@
+; -stop-after would normally dump MIR, but with -filetype=null as well check
+; there's no output at all.
+; RUN: llc -filetype=null -stop-after=finalize-isel -o - %s | count 0