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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I think some of the overloads for constructing an instruction aren't quite right: in some cases, we require a non-null insertion-point so we can retrieve the DataLayout from the insertion-point.  (Particularly instructions that have an explicitly specified alignment.)  In those cases, we shouldn't default to "= nullptr": a null insertion point is guaranteed to assert at runtime.

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