[llvm-exegesis] Error instead of aborting on verification failure (#137581)

This patch makes llvm-exegesis emit an error when the machine function
fails in MachineVerification rather than aborting. This allows
downstream users (particularly https://github.com/google/gematria) to
handle these errors rather than having the entire process crash. This
essentially be NFC from the user perspective minus the addition of the
new error message.

llvm/tools/llvm-exegesis/lib/Assembler.cpp

@@ -29,6 +29,7 @@
 #include "llvm/MC/MCInstrInfo.h"
 #include "llvm/Object/SymbolSize.h"
 #include "llvm/Support/Alignment.h"
+#include "llvm/Support/Error.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/raw_ostream.h"
 
@@ -323,10 +324,8 @@ Error assembleToStream(const ExegesisTarget &ET,
   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 :
-       {"postrapseudos", "machineverifier", "prologepilog"})
+  for (const char *PassName : {"postrapseudos", "prologepilog"})
     if (addPass(PM, PassName, *TPC))
       return make_error<Failure>("Unable to add a mandatory pass");
   TPC->setInitialized();
@@ -337,6 +336,10 @@ Error assembleToStream(const ExegesisTarget &ET,
     return make_error<Failure>("Cannot add AsmPrinter passes");
 
   PM.run(*Module); // Run all the passes
+  bool MFWellFormed =
+      MF.verify(nullptr, "llvm-exegesis Assembly", &outs(), false);
+  if (!MFWellFormed)
+    return make_error<Failure>("The machine function failed verification.");
   return Error::success();
 }