[SDAG] Don't move DBG_VALUE instructions after insertion point during scheduling (PR53243)

EmitSchedule() shouldn't be touching instructions after the provided
insertion point. The change introduced in D83561 performs a scan to
the end of the block, and thus may move unrelated instructions. In
particular, this ends up moving instructions that have been produced
by FastISel and will later be deleted. Moving them means that more
instructions than intended are removed.

Fix this by stopping the iteration when the insertion point is
reached.

Fixes https://github.com/llvm/llvm-project/issues/53243.

Differential Revision: https://reviews.llvm.org/D117489