Introduce CPUStringIsValid() into MCSubtargetInfo and use it for ARM .cpu parsing.

Previously .cpu directive in ARM assembler didnt switch to the new CPU and
therefore acted as a nop. This implemented real action for .cpu and eg. 
allows to assembler FreeBSD kernel with -integrated-as.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223147 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCSubtargetInfo.h

@@ -136,6 +136,15 @@ public:
 
   /// Initialize an InstrItineraryData instance.
   void initInstrItins(InstrItineraryData &InstrItins) const;
+
+  /// Check whether the CPU string is valid.
+  bool CPUStringIsValid(StringRef CPU) {
+    auto Found = std::find_if(ProcDesc.begin(), ProcDesc.end(),
+                              [=](const SubtargetFeatureKV &KV) {
+                                return CPU == KV.Key; 
+                              });
+    return Found != ProcDesc.end();
+  }
 };
 
 } // End llvm namespace