[CGP] Fix the GV handling in complex addressing mode

If in complex addressing mode the difference is in GV then
base reg should not be installed because we plan to use
base reg as a merge point of different GVs.

This is a fix for PR35980.

Reviewers: reames, john.brawn, santosh
Reviewed By: john.brawn
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D42230

llvm-svn: 323192