[InstCombine][SVE] Fix visitGetElementPtrInst for scalable type.

Summary:
This patch fix the following issues in InstCombiner::visitGetElementPtrInst

1. Skip for scalable type if transformation requires fixed size number of vector element.
2. Skip for scalable type if transformation relies on compile-time known type alloc size.
3. Use VectorType::getElementCount when scalable property is used to construct new VectorType.
4. Use TypeSize::getKnownMinSize when minimal size of a scalable type is valid to determine GEP 'inbounds'.
5. Explicitly call TypeSize::getFixedSize to avoid implicit type conversion to uint64_t.

Reviewers: sdesmalen, efriedma, spatel, ctetreau

Reviewed By: efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78081