[SVE][CodeGen] Replace more TypeSize comparison operators with their scalar equivalents

In certain places in llvm/lib/CodeGen we were relying upon the TypeSize
comparison operators when in fact the code was only ever expecting
either scalar values or fixed width vectors. This patch changes a few
functions that were always expecting to work on scalar or fixed width
types:

1. DAGCombiner::mergeTruncStores - deals with scalar integers only.
2. DAGCombiner::ReduceLoadWidth - not valid for vectors.
3. DAGCombiner::createBuildVecShuffle - should only be used for
   fixed width vectors.
4. SelectionDAGLegalize::ExpandFCOPYSIGN and
   SelectionDAGLegalize::getSignAsIntValue - only work on scalars.

Differential Revision: https://reviews.llvm.org/D88562