[AMDGPU] performAddCombine should run after DAG is legalized.

Summary: performAddCombine should run after DAG is legalized; Otherwise generic optimization
         in the DAGCombiner can optimize an addcarry+trunc into an addcarry instruction with
         illegal types.

Author: FarhanaAleen

Reviewed By: rampitec

Subscribers: llvm-commits, AMDGPU

Differential Revision: https://reviews.llvm.org/D46337

llvm-svn: 331368