[SLP]Improve isGatherShuffledEntry by looking deeper through the reused scalars.

The compiler may produce better results if it does not look for
constants, uses an extra analysis of phi nodes, looks through all tree
nodes without skipping the cases, where the very first set of nodes is
empty. Also, it tries to reshufle the nodes if it is profitable for
sure, i.e. at least 2 scalars are used for single node permutation and at
least 3 scalars are used for the permutation of 2 nodes.

Part of D110978

Differential Revision: https://reviews.llvm.org/D141512