[SDAG] Avoid deleted SDNodes PromoteIntBinOp

Reorder work in PromoteIntBinOp to prevent stale (deleted) nodes from
being used.

Fixes PR32340 and PR32345.

Reviewers: hfinkel, dbabokin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31148

llvm-svn: 298923