[AMDGPU] Fix assertion due to assuming pointer in default addr space is 32 bit

The backend assumes pointer in default addr space is 32 bit, which is not
true for the new addr space mapping and causes assertion for unresolved
functions.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D39643


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317476 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed