[ValueTracking] Avoid blind cast from Operator to Instruction

Summary:
Avoid blind cast from Operator to ExtractElementInst in
computeKnownBitsFromOperator. This resulted in some crashes
in downstream fuzzy testing. Instead we use getOperand directly
on the Operator when accessing the vector/index operands.

Haven't seen any problems with InsertElement and ShuffleVector,
but I believe those could be used in constant expressions as well.
So the same kind of fix as for ExtractElement was also applied for
InsertElement.

When it comes to ShuffleVector we now simply bail out if a dynamic
cast of the Operator to ShuffleVectorInst fails. I've got no
reproducer indicating problems for ShuffleVector, and a fix would be
slightly more complicated as getShuffleDemandedElts is involved.

Reviewers: RKSimon, nikic, spatel, efriedma

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76564