[AArch64] Allow poison elements of fixed-vectors to be duplicated as a widened element

Expanding upon https://reviews.llvm.org/D138203, allow null indices in
InsertElts to be matched with any value and be duplicated if the fixed
vector the scalar values are inserted into is poison, and the scalable vector
the subvector being inserted into is poison.

Differential Revision: https://reviews.llvm.org/D141846