[SLP] Rework `findBuildAggregate()` from ercursive form to iterative, NFC.

Reviewers: mkuper

Subscribers: llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D30103

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295641 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed