[VirtualInstruction] Avoid use of getStmtFor(BB). NFC.

With this patch, we get rid of the last use of getStmtFor(BB). Here
this is done by getting the last statement of the incoming block in
case the user is a phi node; otherwise just fetching the statement
comprising the instruction for which the virtual use is being created.

Differential Revision: https://reviews.llvm.org/D36268

llvm-svn: 309947