[CGP] Check for existing inttotpr before creating new one

Make sure CodeGenPrepare doesn't emit multiple inttoptr instructions of
the same integer value while sinking address computations, but rather
CSEs them on the fly: excessive inttoptr's confuse SCEV into thinking
that related pointers have nothing to do with each other.

This problem blocks LoadStoreVectorizer from vectorizing some of the
loads / stores in a downstream target.

Reviewed By: hfinkel

Differential Revision: https://reviews.llvm.org/D56838

llvm-svn: 351582