[llvm] [llvm] Fix ObjectSizeOffsetVisitor behavior in exact mode upon negati… (PR #116955)

[Nathan Chancellor via llvm-commits]

nathanchance wrote:

Hmmm, applying this on top of current main (32913724acf9e02beed46999fee1424086b8c884) and doing my full set of builds, I actually see a new crash:

```
llvm/lib/Analysis/MemoryBuiltins.cpp:569: APInt getSizeWithOverflow(const SizeOffsetAPInt &): Assertion `!Offset.isNegative() && "size for a pointer before the allocated object is ambiguous"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: clang-20 … io_uring/kbuf.c
1.      <eof> parser at end of file
2.      Code generation
3.      Running pass 'Function Pass Manager' on module '/home/nathan/cbl/src/clean/linux/io_uring/kbuf.c'.
4.      Running pass 'Split GEPs to a variadic base and a constant offset for better CSE' on function '@io_provide_buffers'
…
```

It’s quite late for me, so I will try to get a reduced reproducer as soon as I am able to tomorrow.

https://github.com/llvm/llvm-project/pull/116955