[RISCV] Use map::count instead of hasExtension in RISCVISAInfo::updateCombination. NFC

hasExtension check isSupportedExtension before the map lookup. All
of the extensions we check for in updateCombination should be valid
extension names so we can bypass that to save some time.

Author: topperc

llvm/lib/TargetParser/RISCVISAInfo.cpp

@@ -890,7 +890,7 @@ void RISCVISAInfo::updateCombination() {
   do {
     MadeChange = false;
     for (StringRef CombineExt : CombineIntoExts) {
-      if (hasExtension(CombineExt))
+      if (Exts.count(CombineExt.str()))
         continue;
 
       // Look up the extension in the ImpliesExt table to find everything it
@@ -899,7 +899,7 @@ void RISCVISAInfo::updateCombination() {
                                     std::end(ImpliedExts), CombineExt);
       bool HasAllRequiredFeatures = std::all_of(
           Range.first, Range.second, [&](const ImpliedExtsEntry &Implied) {
-            return hasExtension(Implied.ImpliedExt);
+            return Exts.count(Implied.ImpliedExt);
           });
       if (HasAllRequiredFeatures) {
         auto Version = findDefaultVersion(CombineExt);