[SLP] Fix for PR31847: Assertion failed: (isLoopInvariant(Operands[i], L) && "SCEVAdd...

[SLP] Fix for PR31847: Assertion failed: (isLoopInvariant(Operands[i], L) && "SCEVAddRecExpr operand is not loop-invariant!")

Initially SLP vectorizer replaced all going-to-be-vectorized
instructions with Undef values. It may break ScalarEvaluation and may
cause a crash.
Reworked SLP vectorizer so that it does not replace vectorized
instructions by UndefValue anymore. Instead vectorized instructions are
marked for deletion inside if BoUpSLP class and deleted upon class
destruction.

Reviewers: mzolotukhin, mkuper, hfinkel, RKSimon, davide, spatel

Subscribers: RKSimon, Gerolf, anemet, hans, majnemer, llvm-commits, sanjoy

Differential Revision: https://reviews.llvm.org/D29641

llvm-svn: 373166