[DependenceInfo] change name Write to MustWrite to remove ambiguity [NFC]

"Write" is an overloaded term. In collectInfo() till buildFlow(), it is
used to mean "must writes". However, within the memory based analysis,
it is used to mean "both may and must writes". Renaming the Write
variable helps clarify this difference.

Reviewers: grosser

Tags: #polly

Differential Revision: https://reviews.llvm.org/D31181

llvm-svn: 298361