[SLP]Fix PR57447: Assertion `!getTreeEntry(V) && "Scalar already in tree!"' failed.

The pointer operands for the ScatterVectorize node may contain
non-instruction values and they are not checked for "already being
vectorized". Need to check that such pointers are already vectorized and
gather them instead of trying to build vectorize node to avoid compiler
crash.

Differential Revision: https://reviews.llvm.org/D132949