[NFC][RemoveDIs] Always use iterators for inserting PHIs

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.

GitOrigin-RevId: f33f66be7dc586a597437d7ce7619d87e8637209
1 file changed