Fix APInt bit size in processDbgDeclares

processDbgDeclares assumes pointer size is the same for different addr spaces.
It uses pointer size for addr space 0 for all pointers, which causes assertion
in stripAndAccumulateInBoundsConstantOffsets for amdgcn---amdgiz since
pointer in addr space 5 has different size than in addr space 0.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D40085

llvm-svn: 318370