[NFC][IR] Value: assert this->takeName(this)

Need to add an assert about this->takeName(this). This restriction is already documented, so this is just an NFC check.
Without this assertion (as prescribed by original comments for this API), name deletion or down-stream assert failures may occur in other routines: e.g. at the beginning of replaceAllUsesWith() below.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D119636