[RISCV] Replace call to APInt::countTrailingZeros with uint64_t verso…

…n. NFC

We know the number of bits is 64 or 32 so we can use the uint64_t
version directly. This saves the APInt needing to check for the
small vs large size.

llvm/lib/Target/RISCV/RISCVInstrInfoZb.td

@@ -83,13 +83,13 @@ def shfl_uimm : Operand<XLenVT>, ImmLeaf<XLenVT, [{
 
 def BCLRXForm : SDNodeXForm<imm, [{
   // Find the lowest 0.
-  return CurDAG->getTargetConstant(N->getAPIntValue().countTrailingOnes(),
+  return CurDAG->getTargetConstant(countTrailingOnes(N->getZExtValue()),
                                    SDLoc(N), N->getValueType(0));
 }]>;
 
 def BSETINVXForm : SDNodeXForm<imm, [{
   // Find the lowest 1.
-  return CurDAG->getTargetConstant(N->getAPIntValue().countTrailingZeros(),
+  return CurDAG->getTargetConstant(countTrailingZeros(N->getZExtValue()),
                                    SDLoc(N), N->getValueType(0));
 }]>;