[SLP]Delay vectorization of postponable values for instructions with no users.

[SLP]Delay vectorization of postponable values for instructions with no users.

SLP vectorizer tries to find the reductions starting the operands of the
instructions with no-users/void returns/etc. But such operands can be
postponable instructions, like Cmp, InsertElement or InsertValue. Such
operands still must be postponed, vectorizer should not try to vectorize
them immediately.

Differential Revision: https://reviews.llvm.org/D131965