[InstCombine] PR37603: low bit mask canonicalization

Summary:
This is [[ https://bugs.llvm.org/show_bug.cgi?id=37603 | PR37603 ]].

https://godbolt.org/g/VCMNpS
https://rise4fun.com/Alive/idM

When doing bit manipulations, it is quite common to calculate some bit mask,
and apply it to some value via `and`.

The typical C code looks like:
```
int mask_signed_add(int nbits) {
    return (1 << nbits) - 1;
}
```
which is translated into (with `-O3`)
```
define dso_local i32 @mask_signed_add(int)(i32) local_unnamed_addr #0 {
  %2 = shl i32 1, %0
  %3 = add nsw i32 %2, -1
  ret i32 %3
}
```

But there is a second, less readable variant:
```
int mask_signed_xor(int nbits) {
    return ~(-(1 << nbits));
}
```
which is translated into (with `-O3`)
```
define dso_local i32 @mask_signed_xor(int)(i32) local_unnamed_addr #0 {
  %2 = shl i32 -1, %0
  %3 = xor i32 %2, -1
  ret i32 %3
}
```

Since we created such a mask, it is quite likely that we will use it in `and` next.
And then we may get rid of `not` op by folding into `andn`.

But now that i have actually looked:
https://godbolt.org/g/VTUDmU
_some_ backend changes will be needed too.
We clearly loose `bzhi` recognition.

Reviewers: spatel, craig.topper, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47428

llvm-svn: 334127