[DAGCombiner] In mergeTruncStore, make sure we aren't storing shifted in bits. (llvm#90939)

When looking through a right shift, we need to make sure that all of
the bits we are using from the shift come from the shift input and
not the sign or zero bits that are shifted in.
    
Fixes llvm#90936.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -8840,6 +8840,10 @@ SDValue DAGCombiner::mergeTruncStores(StoreSDNode *N) {
       if (ShiftAmtC % NarrowNumBits != 0)
         return SDValue();
 
+      // Make sure we aren't reading bits that are shifted in.
+      if (ShiftAmtC > WideVal.getScalarValueSizeInBits() - NarrowNumBits)
+        return SDValue();
+
       Offset = ShiftAmtC / NarrowNumBits;
       WideVal = WideVal.getOperand(0);
     }

llvm/test/CodeGen/AArch64/pr90936.ll

@@ -3,8 +3,10 @@
 
 define void @f(i16 %arg, ptr %arg1) {
 ; CHECK-LABEL: f:
-; CHECK:       // %bb.0: // %bb
-; CHECK-NEXT:    strh w0, [x1]
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ubfx w8, w0, #8, #6
+; CHECK-NEXT:    strb w0, [x1]
+; CHECK-NEXT:    strb w8, [x1, #1]
 ; CHECK-NEXT:    ret
 bb:
   %i = trunc i16 %arg to i8