llvm-mca: Error on MCSubtargetInfo construction failure (#159215)

Replace assert with a handled error. The same should probably
be done for all of the MC constructors.

Author: arsenm

llvm/tools/llvm-mca/llvm-mca.cpp

@@ -413,7 +413,11 @@ int main(int argc, char **argv) {
 
   std::unique_ptr<MCSubtargetInfo> STI(
       TheTarget->createMCSubtargetInfo(TheTriple, MCPU, FeaturesStr));
-  assert(STI && "Unable to create subtarget info!");
+  if (!STI) {
+    WithColor::error() << "unable to create subtarget info\n";
+    return 1;
+  }
+
   if (!STI->isCPUStringValid(MCPU))
     return 1;