[RISCV] Improve BCLRITwoBitsMaskHigh SDNodeXForm. NFC

Use getZExtValue instead of getSExtValue since we are going to
overwrite the sign extension on RV32. getZExtValue should be cheaper than
getSExtValue.

Use maskLeadingOnes to improve readability.

Author: topperc

llvm/lib/Target/RISCV/RISCVInstrInfoZb.td

@@ -124,9 +124,9 @@ def BCLRITwoBitsMaskLow : SDNodeXForm<imm, [{
 }]>;
 
 def BCLRITwoBitsMaskHigh : SDNodeXForm<imm, [{
-  uint64_t I = N->getSExtValue();
+  uint64_t I = N->getZExtValue();
   if (!Subtarget->is64Bit())
-    I |= 0xffffffffull << 32;
+    I |= maskLeadingOnes<uint64_t>(32);
   return CurDAG->getTargetConstant(llvm::Log2_64(~I), SDLoc(N),
                                    N->getValueType(0));
 }]>;