BlockGenerator: inline lookupAvailableValue into getValue [NFC]

There was no good reason why this code was split accross two functions.

In subsequent changes we will change the order in which values are looked up.
Doing so would make the split into two functions even more arbitrary.

We also slightly improve the documentation.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@221388 91177308-0d34-0410-b5e6-96231b3b80d8