[VectorCombine] make cost calc consistent for binops and cmps

Code duplication (subsequently removed by refactoring) allowed
a logic discrepancy to creep in here.

We were being conservative about creating a vector binop -- but
not a vector cmp -- in the case where a vector op has the same
estimated cost as the scalar op. We want to be more aggressive
here because that can allow other combines based on reduced
instruction count/uses.

We can reverse the transform in DAGCombiner (potentially with a
more accurate cost model) if this causes regressions.

AFAIK, this does not conflict with InstCombine. We have a
scalarize transform there, but it relies on finding a constant
operand or a matching insertelement, so that means it eliminates
an extractelement from the sequence (so we won't have 2 extracts
by the time we get here if InstCombine succeeds).

Differential Revision: https://reviews.llvm.org/D75062