Change the context instruction for computeKnownBits in LoadStoreVectorizer pass

This change enables cases for which the index value for the first
load/store instruction in a pair could be a function argument. This
allows using llvm.assume to provide known bits information in such
cases.

Patch by Viacheslav Nikolaev. Thanks!

Differential Revision: https://reviews.llvm.org/D101680