[X86] LowerShift - directly initialize SmallVector with build vector operands. NFC.

Don't push_back the operands separately.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -30125,11 +30125,7 @@ static SDValue LowerShift(SDValue Op, const X86Subtarget &Subtarget,
     // element to the other.
     // This optimized lowering is only valid if the elements in a pair can
     // be treated identically.
-    SmallVector<SDValue, 32> AmtWideElts;
-    AmtWideElts.reserve(NumElts);
-    for (unsigned I = 0; I != NumElts; ++I) {
-      AmtWideElts.push_back(Amt.getOperand(I));
-    }
+    SmallVector<SDValue, 32> AmtWideElts(Amt->op_begin(), Amt->op_end());
     SmallVector<SDValue, 32> TmpAmtWideElts;
     int WideEltSizeInBits = EltSizeInBits;
     while (WideEltSizeInBits < 32) {