Start moving away from insert-before Instruction pointers

[svenvh]

In preparation for LLVM's deprecation of Instruction constructors and Create methods that accept Instruction pointers, upgrade call sites to pass an iterator instead of an Instruction pointer.

This patch only upgrades constructor and Create calls that were trivial to upgrade with no impact on the test suite. Some calls remain to be upgraded as future work.

[asudarsa]

Thanks for looking into this @svenvh One quick comment (might be my ignorance). Do we need a nullptr check before dereferencing these pointers?

Thanks

[svenvh]

Yes, in most if not all cases, the variable is already dereferenced for getName, getOperand or replaceUsesOfWith calls.