[llvm] [VectorCombine] Try to scalarize vector loads feeding bitcast instructions. (PR #164682)

[Julian Nagele via llvm-commits]

juliannagele wrote:

> I'd be much easier with the patch if it was merely:
> 
> ```
> m_BitCast(m_OneUse(m_Load()))
> ```
> 
> I'm not clear on the need for iterating across users etc.

It was just straight-forward to support multiple users. The load-ext scalarization is looking at all users already anyway so just checking whether they're all bitcasts at the same time seemed natural and I didn't feel like restricting it more than the other load scalarization was simpler. That being said, one user should catch the case we're after, so if you strongly prefer that it'd be ok to change it.

https://github.com/llvm/llvm-project/pull/164682