[clang] [llvm] [polly] [LLVM] Add InsertPosition union-type to remove overloads of Instruction-creation (PR #94226)

[Stephen Tozer via cfe-commits]

SLTozer wrote:

> I think some of the overloads for constructing an instruction aren't quite right:

That's reasonable - I defaulted to adding nullptr-defaults in the case where it didn't cause incorrect overloads, but it's easier to keep only the existing defaults and let any others be added later as appropriate; I've updated the patch accordingly.

https://github.com/llvm/llvm-project/pull/94226