[RISCV] Refactor version major version parsing in parseNormalizedArchString. NFC

Use an index variable and array indexing instead of manipulating
a temporary StringRef.

Author: topperc

llvm/lib/TargetParser/RISCVISAInfo.cpp

@@ -460,19 +460,18 @@ RISCVISAInfo::parseNormalizedArchString(StringRef Arch) {
 
     // Split Prefix into the extension name and the major version number
     // (the trailing digits of Prefix).
-    int TrailingDigits = 0;
-    StringRef ExtName = Prefix;
-    while (!ExtName.empty()) {
-      if (!isDigit(ExtName.back()))
+    size_t VersionStart = Prefix.size();
+    while (VersionStart != 0) {
+      if (!isDigit(Prefix[VersionStart - 1]))
         break;
-      ExtName = ExtName.drop_back(1);
-      TrailingDigits++;
+      --VersionStart;
     }
-    if (!TrailingDigits)
+    if (VersionStart == Prefix.size())
       return createStringError(errc::invalid_argument,
                                "extension lacks version in expected format");
 
-    StringRef MajorVersionStr = Prefix.take_back(TrailingDigits);
+    StringRef ExtName = Prefix.slice(0, VersionStart);
+    StringRef MajorVersionStr = Prefix.slice(VersionStart, StringRef::npos);
     if (MajorVersionStr.getAsInteger(10, MajorVersion))
       return createStringError(errc::invalid_argument,
                                "failed to parse major version number");