[RISCV] Make RISCVISAInfo::updateMaxELen extension checking more robust. Add inference from V extension. (#90650)

We weren't fully checking that we parsed Zve*x/f/d correctly. This could
break if new extension is added that starts with Zve.

We were assuming the Zve64d is present whenever V is so we only
inferred from Zve*. It's more correct to infer ELEN from V itself too.

Author: topperc

llvm/lib/TargetParser/RISCVISAInfo.cpp

@@ -929,19 +929,28 @@ void RISCVISAInfo::updateMinVLen() {
 }
 
 void RISCVISAInfo::updateMaxELen() {
+  assert(MaxELenFp == 0 && MaxELen == 0);
+  if (Exts.count("v")) {
+    MaxELenFp = std::max(MaxELenFp, 64u);
+    MaxELen = std::max(MaxELen, 64u);
+  }
+
   // handles EEW restriction by sub-extension zve
   for (auto const &Ext : Exts) {
     StringRef ExtName = Ext.first;
     bool IsZveExt = ExtName.consume_front("zve");
     if (IsZveExt) {
       if (ExtName.back() == 'f')
         MaxELenFp = std::max(MaxELenFp, 32u);
-      if (ExtName.back() == 'd')
+      else if (ExtName.back() == 'd')
         MaxELenFp = std::max(MaxELenFp, 64u);
+      else if (ExtName.back() != 'x')
+        continue;
+
       ExtName = ExtName.drop_back();
       unsigned ZveELen;
-      ExtName.getAsInteger(10, ZveELen);
-      MaxELen = std::max(MaxELen, ZveELen);
+      if (!ExtName.getAsInteger(10, ZveELen))
+        MaxELen = std::max(MaxELen, ZveELen);
     }
   }
 }