The previous statement removed all uses.

The code was checking the gep ptr type as opposed to the gep source element
type in calculating the offset scale.

Fixes #120993

Identified with misc-include-cleaner.

Similar to https://github.com/llvm/llvm-project/pull/96902, this adds
`getDataLayout()` helpers to Function and GlobalValue, replacing the
current `getParent()->getDataLayout()` pattern.

These are the last remaining "trivial" changes to passes that use
Instruction pointers for insertion. All of this should be NFC, it's just
changing the spelling of how we identify a position.

In one or two locations, I'm also switching uses of getNextNode etc to
using std::next with iterators. This too should be NFC.

---------

Merged by: Stephen Tozer <stephen.tozer@sony.com>

It's becoming potentially unsafe to insert a PHI instruction using a plain
Instruction pointer. Switch all the remaining sites that create and insert
PHIs to use iterators instead. For example, the code in
ComplexDeinterleavingPass.cpp is definitely at-risk of mixing PHIs and
debug-info.

With commuted operands on the phi node, the two old incoming values
could be removed in the wrong order, removing newly added operand
instead of the old one.