[SelectionDAGBuilder] Simplify visitShift. NFC

This code was detecting whether the value returned by getShiftAmountTy
can represent all shift amounts. If not, it would use MVT::i32 as a
placeholder. getShiftAmountTy was updated last year to return i32
if the type returned by the target couldn't represent all values.

This means the MVT::i32 case here is dead and can the logic can
be simplified.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D120164

Author: topperc

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -3150,26 +3150,12 @@ void SelectionDAGBuilder::visitShift(const User &I, unsigned Opcode) {
   EVT ShiftTy = DAG.getTargetLoweringInfo().getShiftAmountTy(
       Op1.getValueType(), DAG.getDataLayout());
 
-  // Coerce the shift amount to the right type if we can.
+  // Coerce the shift amount to the right type if we can. This exposes the
+  // truncate or zext to optimization early.
   if (!I.getType()->isVectorTy() && Op2.getValueType() != ShiftTy) {
-    unsigned ShiftSize = ShiftTy.getSizeInBits();
-    unsigned Op2Size = Op2.getValueSizeInBits();
-    SDLoc DL = getCurSDLoc();
-
-    // If the operand is smaller than the shift count type, promote it.
-    if (ShiftSize > Op2Size)
-      Op2 = DAG.getNode(ISD::ZERO_EXTEND, DL, ShiftTy, Op2);
-
-    // If the operand is larger than the shift count type but the shift
-    // count type has enough bits to represent any shift value, truncate
-    // it now. This is a common case and it exposes the truncate to
-    // optimization early.
-    else if (ShiftSize >= Log2_32_Ceil(Op1.getValueSizeInBits()))
-      Op2 = DAG.getNode(ISD::TRUNCATE, DL, ShiftTy, Op2);
-    // Otherwise we'll need to temporarily settle for some other convenient
-    // type.  Type legalization will make adjustments once the shiftee is split.
-    else
-      Op2 = DAG.getZExtOrTrunc(Op2, DL, MVT::i32);
+    assert(ShiftTy.getSizeInBits() >= Log2_32_Ceil(Op1.getValueSizeInBits()) &&
+           "Unexpected shift type");
+    Op2 = DAG.getZExtOrTrunc(Op2, getCurSDLoc(), ShiftTy);
   }
 
   bool nuw = false;