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

Summary:
This patch fixes the following issues in visitInsertElementInst:

      1. Bail out for scalable type when analysis requires fixed size number of vector elements.
      2. Use cast<FixedVectorType> to get vector number of elements. This ensure assertion
          on scalable vector type.
      3. For scalable type, avoid folding a chain of insertelement into splat:
            insertelt(insertelt(insertelt(insertelt X, %k, 0), %k, 1), %k, 2) ...
              ->
            shufflevector(insertelt(X, %k, 0), undef, zero)
          The length of scalable vector is unknown at compile-time, therefore we don't know if
          given insertelement sequence is valid for splat.

Reviewers: sdesmalen, efriedma, spatel, nikic

Reviewed By: sdesmalen, efriedma

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78895