CGP: Clear data structures at the end of a loop iteration instead of the beginning.

Clearing LargeOffsetGEPMap at the end fixes a bug where if a large
offset GEP is in a dead basic block, we fail an assertion when trying
to delete the block due to the asserting VH in LargeOffsetGEPMap.

Differential Revision: https://reviews.llvm.org/D53464

llvm-svn: 345082