Fix build with LLVM 20.

[hvdijk]

Overview

Fix build with LLVM 20.

Reason for change

• LLVM 20 renames Intrinsic::getDeclaration to Intrinsic::getOrInsertDeclaration.
• LLVM 20 deprecates instruction creation overloads that take an Instruction * as an insertion point, recommending the ones that take an iterator instead. In LLVM 18, for the most part, those overloads taking iterators do not yet exist.

Description of change

• Handle the Intrinsic function rename with a multi_llvm::GetOrInsertIntrinsicDeclaration helper function.
• In LLVM 18, Instruction::insertBefore does already take iterators, so rework the code to consistently use that. This removes some bitcast instructions in memory_operations.cpp that could not be updated, but had become unnecessary ever since LLVM switched to opaque pointers.

Anything else we should know?

If there's any other relevant information we should know that may help us in
understanding and verifying your patch, please include it here.

Checklist

• Read and follow the project Code of Conduct.
• Make sure the project builds successfully with your changes.
• Run relevant testing locally to avoid regressions.
• Run clang-format-19 on all modified code.