SLPVectorizer: Make store chain finding more aggressive with GetUnderlyingObject.

This recursively strips all GEPs like the existing code. It also handles bitcasts and
other operations that do not change the pointer value.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191847 91177308-0d34-0410-b5e6-96231b3b80d8