[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, similar to D85725.

Fix https://gcc.godbolt.org/z/o561P9zj4

Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D158631