[DAGCombiner] Use getShiftAmountConstant in DAGCombiner::foldSelectOfConstants.

This enables fshl to be matched earlier on X86

  %6 = lshr i32 %3, 1
  %7 = select i1 %4, i32 -2147483648, i32 0
  %8 = or i32 %6, %7

X86 uses i8 for shift amounts. SelectionDAGBuilder creates the
ISD::SRL with an i8 shift type. DAGCombiner turns the select into
an ISD::SHL. Prior to this patch it would use i32 for the shift
amount. fshl matching failed because the shift amounts have different
types. LegalizeDAG fixes the ISD::SHL shift amount to i8. This
allowed fshl matching to succeed.

With this patch, the ISD::SHL will be created with an i8 shift
amount. This allows the fshl to match immediately.

No test case beause we still end up with a fshl either way.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -9765,7 +9765,8 @@ SDValue DAGCombiner::foldSelectOfConstants(SDNode *N) {
       if (C1Val.isPowerOf2() && C2Val.isZero()) {
         if (VT != MVT::i1)
           Cond = DAG.getNode(ISD::ZERO_EXTEND, DL, VT, Cond);
-        SDValue ShAmtC = DAG.getConstant(C1Val.exactLogBase2(), DL, VT);
+        SDValue ShAmtC =
+            DAG.getShiftAmountConstant(C1Val.exactLogBase2(), VT, DL);
         return DAG.getNode(ISD::SHL, DL, VT, Cond, ShAmtC);
       }