[DAGCombiner] Factor out duplicated logic for an extload combine, NFC (5/N)

Part of the logic for combining (zext (load ...)) and (sext (load ...))
is duplicated. This creates problems because bugs in one version have to
be fixed again in the other version.

To address this, as a first step, I've extracted the duplicate logic
into a helper. I'll fix the debug location bug in the helper and
eliminate the copy of its logic in a followup.

Part of: llvm.org/PR37262

Differential Revision: https://reviews.llvm.org/D46157

llvm-svn: 332117

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