[CodeGenPrepare] Clean up dbg.value use-before-def as late as possible

CodeGenPrepare has a strategy for moving dbg.values so that a value's
definition always dominates its debug users. This cleanup was happening
too early (before certain CGP transforms were run), resulting in some
dbg.value use-before-def errors.

Perform this cleanup as late as possible to avoid use-before-def.

llvm-svn: 340370