[GISel]: Fix incorrect IRTranslation while translating null pointer types

https://reviews.llvm.org/D44762

Currently IRTranslator produces
%vreg17<def>(p0) = G_CONSTANT 0;

instead we should build
%vreg16(s64) = G_CONSTANT 0
%vreg17(p0) = G_INTTOPTR %vreg16

reviewed by @aemerson.

llvm-svn: 328218