Run GVN during the cleanup

Currently, GVN can be necessary to eliminate redundant instructions in case
of, for instance, GEMM and float type. This patch makes GVN be run during
the cleanup.

Reviewed-by: Tobias Grosser <tobias@grosser.es&gt;,>
             Michael Kruse <llvm@meinersbur.de>

Differential Revision: https://reviews.llvm.org/D37340

llvm-svn: 312307