[NFC][RemoveDIs] Switch constant-hoisting to insert with iterators (#84738)

Seeing how constant-hoisting also happens to store an insertion-point in
a DenseMap, this means we have to install DenseMapInfo for hashing
BasicBlock::iterators and comparing them. I'm not really sure where to
put the DenseMapInfo declarations as BasicBlock.h seems most logical,
but that then means including DenseMap.h into pretty much all of LLVM.
I've sent this up to the compile time tracker to see whether there's a
major cost from this.

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