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

hasExtension checks if the extension name is a known extension name.
That should always be true for the extensions listed here so we can
skip that check.

Author: topperc

llvm/lib/TargetParser/RISCVISAInfo.cpp

@@ -998,19 +998,19 @@ RISCVISAInfo::postProcessAndChecking(std::unique_ptr<RISCVISAInfo> &&ISAInfo) {
 
 StringRef RISCVISAInfo::computeDefaultABI() const {
   if (XLen == 32) {
-    if (hasExtension("e"))
+    if (Exts.count("e"))
       return "ilp32e";
-    if (hasExtension("d"))
+    if (Exts.count("d"))
       return "ilp32d";
-    if (hasExtension("f"))
+    if (Exts.count("f"))
       return "ilp32f";
     return "ilp32";
   } else if (XLen == 64) {
-    if (hasExtension("e"))
+    if (Exts.count("e"))
       return "lp64e";
-    if (hasExtension("d"))
+    if (Exts.count("d"))
       return "lp64d";
-    if (hasExtension("f"))
+    if (Exts.count("f"))
       return "lp64f";
     return "lp64";
   }