[SLP]Fix PR51320: Try to vectorize single store operands.

Currently, we try to vectorize values, feeding into stores, only if
slp-vectorize-hor-store option is provided. We can safely enable
vectorization of the value operand of a single store in the basic block,
if the operand value is used only in store.
It should enable extra vectorization and should not increase compile
time significantly.
Fixes https://github.com/llvm/llvm-project/issues/51320

Differential Revision: https://reviews.llvm.org/D131894