[SLPVectorizer] Don't include isAssumeLikeIntrinsics in ScheduleRegionSize

We don't want the existence of debug instructions affect codegen so we now
ignore debug instructions and other "isAssumeLikeIntrinsics in the
"extend schedule region" search loop in
BoUpSLP::BlockScheduling::extendSchedulingRegion.

Differential Revision: https://reviews.llvm.org/D152441