[DAGCombiner] generalize binop-of-splats scalarization

If we only match build vectors, we can miss some patterns
that use shuffles as seen in the affected tests.

Note that the underlying calls within getSplatSourceVector()
have the potential for compile-time explosion because of
exponential recursion looking through binop opcodes, but
currently the list of supported opcodes is very limited.
Both of those problems should be addressed in follow-up
patches.

llvm-svn: 358984

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