[NFC] Restore original SmallString size for X86TargetMachine::getSubtargetImpl lookup

Better safe than sorry here, quoting Craig Topper:

> Clang passes a pretty lengthy feature string.

Author: serge-sans-paille

llvm/lib/Target/X86/X86TargetMachine.cpp

@@ -248,7 +248,7 @@ X86TargetMachine::getSubtargetImpl(const Function &F) const {
   StringRef FS =
       FSAttr.isValid() ? FSAttr.getValueAsString() : (StringRef)TargetFS;
 
-  SmallString<64> Key;
+  SmallString<512> Key;
   // The additions here are ordered so that the definitely short strings are
   // added first so we won't exceed the small size. We append the
   // much longer FS string at the end so that we only heap allocate at most