[SVE] Make Constant::getSplatValue work for scalable vector splats

Summary:
Make Constant::getSplatValue recognize scalable vector splats of the
form created by ConstantVector::getSplat. Add unit test to verify that
C == ConstantVector::getSplat(C)->getSplatValue() for fixed width and
scalable vector splats

Reviewers: efriedma, spatel, fpetrogalli, c-rhodes

Reviewed By: efriedma

Subscribers: sdesmalen, tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82416