[TargetLowering][AMDGPU] Make scalarizeVectorLoad return a pair of SDValues...

[TargetLowering][AMDGPU] Make scalarizeVectorLoad return a pair of SDValues instead of creating a MERGE_VALUES node. NFCI

This allows us to clean up some places that were peeking through
the MERGE_VALUES node after the call. By returning the SDValues
directly, we can clean that up.

Unfortunately, there are several call sites in AMDGPU that wanted
the MERGE_VALUES and now need to create their own.