[Support] Add KnownBits::computeForSubBorrow

[RKSimon]

Add the sibling to KnownBits::computeForAddCarry:

• Actual KnownBits implementation is a wrapper to the internal computeForAddCarry, similar to KnownBits::computeForAddCarry, with minor bit twiddling/swapping.
• Add a SubBorrowExhaustive unit test to KnownBitsTest.cpp
• Hook up the USUBO/SSUBO/USUBO_CARRY/SSUBO_CARRY/SUBC cases in SelectionDAG::computeKnownBits to make use of it (similar to the ADD variants already do), with suitable testing.

[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:

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2. Fix the issue locally.
3. Run the test suite locally.
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6. Submit the patch to Phabricator.
   6.1) Detailed instructions can be found here

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[christiankissig]

@RKSimon I'd like to take a stab.

[christiankissig]

PR #67788 @RKSimon

[dlumma]

@christiankissig assigning this ticket to you since it looks like the work is done.