[CSSPGO] Move pseudo probes to the beginning of a block to unblock SelectionDAG combine.
Pseudo probes, when scattered in a block, can be chained dependencies of other regular DAG nodes and block DAG combine optimizations. To fix this, scattered probes in a block are grouped and placed at the beginning of the block. This shouldn't affect the profile quality.
Reviewed By: wenlei, wmi
Differential Revision: https://reviews.llvm.org/D100002