[GVN] Fix VNCoercion for Scalable Vector.

Summary:
For VNCoercion, skip scalable vector when analysis rely on fixed size,
otherwise call TypeSize::getFixedSize() explicitly.

Add unit tests to check funtionality of GVN load elimination for scalable type.

Reviewers: sdesmalen, efriedma, spatel, fhahn, reames, apazos, ctetreau

Reviewed By: efriedma

Subscribers: bjope, hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76944