[llvm] InlineCost: Use index sizes rather than pointer sizes in a few cases. (PR #119326)

[Owen Anderson via llvm-commits]

resistor wrote:

Tagging @arichardson and @jrtc27 
I don't have a test for these, since they only affect cost modeling. 

This is a subset of b0c5e3a782ac1247f3ea17c9ff7d5419a5070102 in downstream CHERI-LLVM which mentioned fixing assertion failures, but the hash it points to doesn't exist in my tree, and I can't reproduce the assertion failures.

All of that said, the behavior in this diff seems self-evidently correct in context.

https://github.com/llvm/llvm-project/pull/119326