[memdep] Switch a function to return true on success instead of false.

[memdep] Switch a function to return true on success instead of false.

This is much more clear and less surprising IMO. It also makes things
more consistent with the increasingly large chunk of LLVM code that
assumes true-on-success.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262826 91177308-0d34-0410-b5e6-96231b3b80d8