[DebugInfo][RemoveDIs] Switch some insertion routines to use iterators (#75330)

As part of RemoveDIs, we need instruction insertion to be done with
iterators rather than instruction pointers, so that we can communicate
some debug-info facts about the position. This patch is an entirely
mechanical replacement of Instruction * with BasicBlock::iterator, plus
using insertBefore to insert some instructions because we don't have
iterator-taking constructors yet.

Sadly it's not NFC because it causes dbg.value intrinsics / their
DPValue equivalents to shift location.