Rollback assumeAligned in MemorySanitizer

Summary: Rollback of parts of D71213. After digging more into the code I think we should leave 0 when creating the instructions (CreateMemcpy, CreateMaskedStore, CreateMaskedLoad). It's probably fine for MemorySanitizer because Alignement is resolved but I'm having a hard time convincing myself it has no impact at all (although tests are passing).

Reviewers: courbet

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71332