[InstCombine] Add tests for invert of lshr (NFC)

Author: nikic

llvm/test/Transforms/InstCombine/free-inversion.ll

@@ -6,6 +6,8 @@ declare i8 @llvm.umin.i8(i8, i8)
 declare i8 @llvm.smax.i8(i8, i8)
 declare i8 @llvm.umax.i8(i8, i8)
 
+declare void @llvm.assume(i1)
+
 declare void @use.i8(i8)
 
 define i8 @xor_1(i8 %a, i1 %c, i8 %x, i8 %y) {
@@ -492,3 +494,33 @@ define i8 @smax_both_freely_invertable_always(i8 %x, i8 %y) {
   %r = call i8 @llvm.smax.i8(i8 %xx, i8 %yy)
   ret i8 %r
 }
+
+define i8 @lshr_nneg(i8 %x, i8 %y) {
+; CHECK-LABEL: @lshr_nneg(
+; CHECK-NEXT:    [[NEG:%.*]] = icmp slt i8 [[X:%.*]], 0
+; CHECK-NEXT:    call void @llvm.assume(i1 [[NEG]])
+; CHECK-NEXT:    [[X_NOT:%.*]] = xor i8 [[X]], -1
+; CHECK-NEXT:    [[SHR:%.*]] = lshr i8 [[X_NOT]], [[Y:%.*]]
+; CHECK-NEXT:    [[SHR_NOT:%.*]] = xor i8 [[SHR]], -1
+; CHECK-NEXT:    ret i8 [[SHR_NOT]]
+;
+  %neg = icmp slt i8 %x, 0
+  call void @llvm.assume(i1 %neg)
+  %x.not = xor i8 %x, -1
+  %shr = lshr i8 %x.not, %y
+  %shr.not = xor i8 %shr, -1
+  ret i8 %shr.not
+}
+
+define i8 @lshr_not_nneg(i8 %x, i8 %y) {
+; CHECK-LABEL: @lshr_not_nneg(
+; CHECK-NEXT:    [[X_NOT:%.*]] = xor i8 [[X:%.*]], -1
+; CHECK-NEXT:    [[SHR:%.*]] = lshr i8 [[X_NOT]], [[Y:%.*]]
+; CHECK-NEXT:    [[SHR_NOT:%.*]] = xor i8 [[SHR]], -1
+; CHECK-NEXT:    ret i8 [[SHR_NOT]]
+;
+  %x.not = xor i8 %x, -1
+  %shr = lshr i8 %x.not, %y
+  %shr.not = xor i8 %shr, -1
+  ret i8 %shr.not
+}