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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> @SLTozer That sounds reasonable to me.

Thanks @nikic - I've created new reviews, https://github.com/llvm/llvm-project/pull/94224 and https://github.com/llvm/llvm-project/pull/94226, which collectively implement the first step of this approach. Rather than use an actual union type, the first patch adds new functionality to the ilist classes to allow us to use just a `BasicBlock::iterator` as an insert position in all cases, and the second patch uses that to replace each insert position overload with a single version.

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