BlockGenerator: Split getNewValue.

Split the old getNewValue into two parts:

1. The function "lookupAvailableValue" that return the new version of
the instruction which is already available.

2. The function calls "lookupAvailableValue", and tries to generate
the new version if it is not available yet.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@187114 91177308-0d34-0410-b5e6-96231b3b80d8