[Transforms][SROA] Skip uses of allocas where the type is scalable

When visiting load and store instructions in SROA skip scalable vectors.
This is relevant in the implementation of the 'arm_sve_vector_bits'
attribute that is used to define VLS types, where an alloca of a
fixed-length vector could be bitcasted to scalable. See D85128 for more
information.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D85725