Fix PR35542: Correct adjusting of private reduction variable

The adjustment is calculated with CreatePtrDiff() which returns
the difference in (base) elements. This is passed to CreateGEP()
so make sure that the GEP base has the correct pointer type:
It needs to be a pointer to the base type, not a pointer to a
constant sized array.

Differential Revision: https://reviews.llvm.org/D40911

llvm-svn: 319931