[llvm] [BOLT][heatmap] Produce zoomed-out heatmaps (PR #140153)

[Amir Ayupov via llvm-commits]

aaupov wrote:

@maksfb:

> * If we are producing multiple files by default, we might have to change docs in more places.

Makes sense. I'll update heatmap doc, anything else?

> * I find the power-of-two limitation for the scale unnecessary.

This made it impossible to enter invalid scales. While working on a custom parser, I realized that a better UX would be to allow specifying bucket sizes in natural units (e.g. 4KB, 1MB):
```
--block-size=default_size[,size1,...]
```

If you agree, I'll switch to that and print warnings in case of invalid sizes/rescales.

> * The variance in default scale steps looks a bit unnatural (64, 4, 64 again). I would keep just 64 and 64 making the default sizes 64 bytes, 4 KB, 256 KB.

It's the downstream of bucket size selection. 1MB works really well for XL binaries, to have a high-level view of the whole address space. I'd like to keep 1MB for that reason. We can add 256KB for sure. 

LMK if that sounds good.

https://github.com/llvm/llvm-project/pull/140153