[RemoveDIs] Use iterators for moving PHIs in loop-unroll-and-jam (#83003)

With no debug intrinsics, correctly identifying the start of a block
with iterators becomes important. We need to use the iterator-returning
methods here in loop-unroll-and-jam where we're shifting PHIs around.
Otherwise they can be inserted after debug-info records, leading to
debug-info attached to PHIs, which is ill formed.

Fixes #83000