[llvm] [IR] Make instr-create fns with Instruction *InsertBefore non-default (PR #86132)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams approved this pull request.

Mechanically LGTM

The changes look like they move in the direction agreed upon in the Discourse discussion. Making the `Instruction*` versions not the default makes it easier to deprecate and remove them.

It's a shame that this introduced some almost-duplicated functions in Instruction.cpp - OTOH, we plan to remove one version.

I think it's worth some others having this on their radar too (cc @dwblaikie and @nikic) - I couldn't see this linked in the Discourse thread, it's probably worth dropping a link there if I haven't just missed it.

https://github.com/llvm/llvm-project/pull/86132