[SDAG] Fix zeroExtend assertion error

Move CombineTo preventing deleted node from being returned in
visitZERO_EXTEND.

Fixes PR32284.

Reviewers: RKSimon, bogner

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31254

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298604 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed