[llvm] [SLP]Fix/improve getSpillCost analysis (PR #129258)

[Alexander Kornienko via llvm-commits]

alexfh wrote:

> > We've found a clang crash that seems to be introduced by this commit. The original build is using instrumented FDO + thinlto with a bunch of dependencies. I managed to get a standalone reproducer (hopefully, nothing important was lost during the automated reduction):
> Checked it, unable to reproduce, most probably it was fixed yesterday

Indeed, this issue was fixed in 202137dbead88004c956595106ab6d80d597508c. I'm building clang and running the original non-reduced compilation to confirm that this also fixes it.

https://github.com/llvm/llvm-project/pull/129258