[RISCV] Simplify code in decomposeMulByConstant. NFC (#100946)

We already checked that the type is a scalar integer, so the constant
node should definitely be a ConstantSDNode. We can use cast instead of
dyn_cast.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -21181,37 +21181,37 @@ bool RISCVTargetLowering::shouldSignExtendTypeInLibCall(EVT Type, bool IsSigned)
 bool RISCVTargetLowering::decomposeMulByConstant(LLVMContext &Context, EVT VT,
                                                  SDValue C) const {
   // Check integral scalar types.
-  const bool HasZmmul = Subtarget.hasStdExtZmmul();
   if (!VT.isScalarInteger())
     return false;
 
   // Omit the optimization if the sub target has the M extension and the data
   // size exceeds XLen.
+  const bool HasZmmul = Subtarget.hasStdExtZmmul();
   if (HasZmmul && VT.getSizeInBits() > Subtarget.getXLen())
     return false;
 
-  if (auto *ConstNode = dyn_cast<ConstantSDNode>(C.getNode())) {
-    // Break the MUL to a SLLI and an ADD/SUB.
-    const APInt &Imm = ConstNode->getAPIntValue();
-    if ((Imm + 1).isPowerOf2() || (Imm - 1).isPowerOf2() ||
-        (1 - Imm).isPowerOf2() || (-1 - Imm).isPowerOf2())
-      return true;
+  auto *ConstNode = cast<ConstantSDNode>(C);
+  const APInt &Imm = ConstNode->getAPIntValue();
 
-    // Optimize the MUL to (SH*ADD x, (SLLI x, bits)) if Imm is not simm12.
-    if (Subtarget.hasStdExtZba() && !Imm.isSignedIntN(12) &&
-        ((Imm - 2).isPowerOf2() || (Imm - 4).isPowerOf2() ||
-         (Imm - 8).isPowerOf2()))
-      return true;
+  // Break the MUL to a SLLI and an ADD/SUB.
+  if ((Imm + 1).isPowerOf2() || (Imm - 1).isPowerOf2() ||
+      (1 - Imm).isPowerOf2() || (-1 - Imm).isPowerOf2())
+    return true;
 
-    // Break the MUL to two SLLI instructions and an ADD/SUB, if Imm needs
-    // a pair of LUI/ADDI.
-    if (!Imm.isSignedIntN(12) && Imm.countr_zero() < 12 &&
-        ConstNode->hasOneUse()) {
-      APInt ImmS = Imm.ashr(Imm.countr_zero());
-      if ((ImmS + 1).isPowerOf2() || (ImmS - 1).isPowerOf2() ||
-          (1 - ImmS).isPowerOf2())
-        return true;
-    }
+  // Optimize the MUL to (SH*ADD x, (SLLI x, bits)) if Imm is not simm12.
+  if (Subtarget.hasStdExtZba() && !Imm.isSignedIntN(12) &&
+      ((Imm - 2).isPowerOf2() || (Imm - 4).isPowerOf2() ||
+       (Imm - 8).isPowerOf2()))
+    return true;
+
+  // Break the MUL to two SLLI instructions and an ADD/SUB, if Imm needs
+  // a pair of LUI/ADDI.
+  if (!Imm.isSignedIntN(12) && Imm.countr_zero() < 12 &&
+      ConstNode->hasOneUse()) {
+    APInt ImmS = Imm.ashr(Imm.countr_zero());
+    if ((ImmS + 1).isPowerOf2() || (ImmS - 1).isPowerOf2() ||
+        (1 - ImmS).isPowerOf2())
+      return true;
   }
 
   return false;