[RS4GC] Update and simplify handling of Constants in findBaseDefiningValueOfVector().

Summary:
This is analogous to r256079, which removed an overly strong assertion, and
r256812, which simplified the code by replacing three conditionals by one.

Reviewers: reames

Subscribers: sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D16019

llvm-svn: 257250