[SLPVectorizer] Pass the right type argument to getCmpSelInstrCost()

In getEntryCost(), make the scalar type for a compare instruction that of the
operands, not i1. This is needed in order to call getCmpSelInstrCost() for a
compare in a sensible way, the same way as the LoopVectorizer does.

New test: test/Transforms/SLPVectorizer/SystemZ/SLP-cmp-cost-query.ll

Review: Matthew Simpson
https://reviews.llvm.org/D31601

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300061 91177308-0d34-0410-b5e6-96231b3b80d8