[InstSimplify] simplifyUnsignedRangeCheck(): if we know that X != 0, handle more cases (PR43246)

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

In this particular case, given
```
char* test(char& base, unsigned long offset) {
  return &base + offset;
}
```
it will end up producing something like
https://godbolt.org/z/LK5-iH
which after optimizations reduces down to roughly
```
define i1 @t0(i8* nonnull %base, i64 %offset) {
  %base_int = ptrtoint i8* %base to i64
  %adjusted = add i64 %base_int, %offset
  %non_null_after_adjustment = icmp ne i64 %adjusted, 0
  %no_overflow_during_adjustment = icmp uge i64 %adjusted, %base_int
  %res = and i1 %non_null_after_adjustment, %no_overflow_during_adjustment
  ret i1 %res
}
```
Without D67122 there was no `%non_null_after_adjustment`,
and in this particular case we can get rid of the overhead:

Here we add some offset to a non-null pointer,
and check that the result does not overflow and is not a null pointer.
But since the base pointer is already non-null, and we check for overflow,
that overflow check will already catch the null pointer,
so the separate null check is redundant and can be dropped.

Alive proofs:
https://rise4fun.com/Alive/WRzq

There are more patterns of "unsigned-add-with-overflow", they are not handled here,
but this is the main pattern, that we currently consider canonical,
so it makes sense to handle it.

https://bugs.llvm.org/show_bug.cgi?id=43246

Reviewers: spatel, nikic, vsk

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits, reames

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67332

llvm-svn: 371349