Move the special Phi logic for hung off uses in to User::allocHungOffUses. NFC.

PhiNode's need to allocate space for an array of Use[N] and then BasicBlock*[N].

They had their own allocHungOffUses to handle all of this.  This moves the logic
in to User::allocHungOffUses and PhiNode passes in a bool to say to allocate
the BB* space too.

Reviewed by Duncan Exon Smith.

llvm-svn: 239489