[InstCombine] Add test for incorrect fshr transform (#89338)

Author: nikic

llvm/test/Transforms/InstCombine/fsh.ll

@@ -1001,3 +1001,13 @@ define <2 x i32> @fsh_unary_shuffle_ops_partial_widening(<3 x i32> %x, <2 x i32>
   %r = call <2 x i32> @llvm.fshr.v2i32(<2 x i32> %a, <2 x i32> %b, <2 x i32> %c)
   ret <2 x i32> %r
 }
+
+; FIXME: This is a miscompile.
+define <2 x i32> @fshr_vec_zero_elem(<2 x i32> %x, <2 x i32> %y) {
+; CHECK-LABEL: @fshr_vec_zero_elem(
+; CHECK-NEXT:    [[FSH:%.*]] = call <2 x i32> @llvm.fshl.v2i32(<2 x i32> [[X:%.*]], <2 x i32> [[Y:%.*]], <2 x i32> <i32 30, i32 0>)
+; CHECK-NEXT:    ret <2 x i32> [[FSH]]
+;
+  %fsh = call <2 x i32> @llvm.fshr.v2i32(<2 x i32> %x, <2 x i32> %y, <2 x i32> <i32 2, i32 0>)
+  ret <2 x i32> %fsh
+}