[SLP] Simplify extendSchedulingRegion

This change uses instruction's comesBefore method to simplify the code significantly. There's little compile time concern here because getSpillCost already calls comesBefore on every basic block which contains a vectorization candidate. The only additional times we'll build basic block ordering is when we can't schedule a vector candidate anywhere in the containing block.

Differential Revision: https://reviews.llvm.org/D120364