[InstCombine] (a+b) <= a && (a+b) != 0 -> (0-b) < a (PR43259)

Summary:
This is again motivated by D67122 sanitizer check enhancement.
That patch seemingly worsens `-fsanitize=pointer-overflow`
overhead from 25% to 50%, which strongly implies missing folds.

This pattern isn't exactly what we get there
(strict vs. non-strict predicate), but this pattern does not
require known-bits analysis, so it is best to handle it first.

```
Name: 0
  %adjusted = add i8 %base, %offset
  %not_null = icmp ne i8 %adjusted, 0
  %no_underflow = icmp ule i8 %adjusted, %base
  %r = and i1 %not_null, %no_underflow
=>
  %neg_offset = sub i8 0, %offset
  %r = icmp ugt i8 %base, %neg_offset
```
https://rise4fun.com/Alive/knp

There are 3 other variants of this pattern,
they all will go into InstSimplify:
https://rise4fun.com/Alive/bIDZ

https://bugs.llvm.org/show_bug.cgi?id=43259

Reviewers: spatel, xbolva00, nikic

Reviewed By: spatel

Subscribers: hiraditya, majnemer, vsk, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67846

llvm-svn: 372767

0 files changed, 0 insertions, 0 deletions