Fix StackSafetyAnalysis crash with scalable vector types.

Summary:
Treat scalable allocas as if they have storage size of 0, and
scalable-typed memory accesses as if their range is unlimited.

This is not a proper support of scalable vector types in the analysis -
we can do better, but not today.

Reviewers: vitalybuka

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73394