[SLP] put verifyFunction call behind EXPENSIVE_CHECKS

A severe compile-time slowdown from this call is noted in:
https://llvm.org/PR48689
My naive fix was to put it under LLVM_DEBUG ( 267ff790 ),
but that's not limiting in the way we want.
This is a quick fix (or we could just remove the call completely
and rely on some later pass to discover potentially wrong IR?).
A bigger/better fix would be to improve/limit verifyFunction()
as noted in:
https://llvm.org/PR47712

Differential Revision: https://reviews.llvm.org/D94328