[APInt] Add APInt::clearBits() method

[RKSimon]

Similar to how we have APInt::setBits(unsigned loBit, unsigned hiBit), we require a APInt::clearBits(unsigned loBit, unsigned hiBit) method to zero out bits in the specific range

Noticed on #131989 - but we need something similar in a number of other cases - notably when manipulating DemandedElts masks for INSERT_SUBVECTOR

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

Similar to how we have APInt::setBits(unsigned loBit, unsigned hiBit), we require a APInt::clearBits(unsigned loBit, unsigned hiBit) method to zero out bits in the specific range

Noticed on #131989 - but we need something similar in a number of other cases - notably when manipulating DemandedElts masks for INSERT_SUBVECTOR

[liamsemeria]

Hi I'd like to work on this.
Just to make sure I start on the right track:
I'll add clearBits to llvm/include/llvm/ADT/APInt.h, and I'll also need to add new tests to make sure it works. Is that correct?
Does ADT build by default or do I need to specify it to build separately?
Sorry for the questions I'm new to contributing.
Thanks

[RKSimon]

Yes you need to add unit test coverage to llvm\unittests\ADT\APIntTest.cpp - ninja ADTTests is probably the best way to build the ADT unit tests

[liamsemeria]

ok sounds good

[liamsemeria]

Its all done I'll have the PR up later today