[InstCombine] add tests for (iN X s>> N-1) | Y; NFC

These are for a sibling fold suggested in D111410.
The tests correspond to the 'and' tests added with:
a35673f

llvm/test/Transforms/InstCombine/or.ll

@@ -1385,3 +1385,62 @@ define i32 @test5_use3(i32 %x, i32 %y) {
   %or1 = or i32 %xor, %neg
   ret i32 %or1
 }
+
+define i8 @ashr_bitwidth_mask(i8 %x, i8 %y) {
+; CHECK-LABEL: @ashr_bitwidth_mask(
+; CHECK-NEXT:    [[SIGN:%.*]] = ashr i8 [[X:%.*]], 7
+; CHECK-NEXT:    [[R:%.*]] = or i8 [[SIGN]], [[Y:%.*]]
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %sign = ashr i8 %x, 7
+  %r = or i8 %sign, %y
+  ret i8 %r
+}
+
+define <2 x i8> @ashr_bitwidth_mask_vec_commute(<2 x i8> %x, <2 x i8> %py) {
+; CHECK-LABEL: @ashr_bitwidth_mask_vec_commute(
+; CHECK-NEXT:    [[Y:%.*]] = mul <2 x i8> [[PY:%.*]], <i8 42, i8 2>
+; CHECK-NEXT:    [[SIGN:%.*]] = ashr <2 x i8> [[X:%.*]], <i8 7, i8 7>
+; CHECK-NEXT:    [[R:%.*]] = or <2 x i8> [[Y]], [[SIGN]]
+; CHECK-NEXT:    ret <2 x i8> [[R]]
+;
+  %y = mul <2 x i8> %py, <i8 42, i8 2>      ; thwart complexity-based ordering
+  %sign = ashr <2 x i8> %x, <i8 7, i8 7>
+  %r = or <2 x i8> %y, %sign
+  ret <2 x i8> %r
+}
+
+define i32 @ashr_bitwidth_mask_use(i32 %x, i32 %y) {
+; CHECK-LABEL: @ashr_bitwidth_mask_use(
+; CHECK-NEXT:    [[SIGN:%.*]] = ashr i32 [[X:%.*]], 7
+; CHECK-NEXT:    call void @use(i32 [[SIGN]])
+; CHECK-NEXT:    [[R:%.*]] = or i32 [[SIGN]], [[Y:%.*]]
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %sign = ashr i32 %x, 7
+  call void @use(i32 %sign)
+  %r = or i32 %sign, %y
+  ret i32 %r
+}
+
+define i8 @ashr_not_bitwidth_mask(i8 %x, i8 %y) {
+; CHECK-LABEL: @ashr_not_bitwidth_mask(
+; CHECK-NEXT:    [[SIGN:%.*]] = ashr i8 [[X:%.*]], 6
+; CHECK-NEXT:    [[R:%.*]] = or i8 [[SIGN]], [[Y:%.*]]
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %sign = ashr i8 %x, 6
+  %r = or i8 %sign, %y
+  ret i8 %r
+}
+
+define i8 @lshr_bitwidth_mask(i8 %x, i8 %y) {
+; CHECK-LABEL: @lshr_bitwidth_mask(
+; CHECK-NEXT:    [[SIGN:%.*]] = lshr i8 [[X:%.*]], 7
+; CHECK-NEXT:    [[R:%.*]] = or i8 [[SIGN]], [[Y:%.*]]
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %sign = lshr i8 %x, 7
+  %r = or i8 %sign, %y
+  ret i8 %r
+}