[InstSimplify][SVE] Fix SimplifyInsert/ExtractElementInst for scalable vector.

Summary:
For scalable vector, index out-of-bound can not be determined at compile-time.
The same apply for VectorUtil findScalarElement().

Add test cases to check the functionality of SimplifyInsert/ExtractElementInst for scalable vector.

Reviewers: sdesmalen, efriedma, spatel, apazos

Reviewed By: efriedma

Subscribers: cameron.mcinally, tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75782