WholeProgramDevirt: For VCP use a 32-bit ConstantInt for the byte offset.

A future change will cause this byte offset to be inttoptr'd and then exported
via an absolute symbol. On the importing end we will expect the symbol to be
in range [0,2^32) so that it will fit into a 32-bit relocation. The problem
is that on 64-bit architectures if the offset is negative it will not be in
the correct range once we inttoptr it.

This change causes us to use a 32-bit integer so that it can be inttoptr'd
(which zero extends) into the correct range.

Differential Revision: https://reviews.llvm.org/D30016

llvm-svn: 295487