[SVE][CodeGen][DAGCombiner] Fix TypeSize warning in redundant store elimination

The modified code in visitSTORE was missing a scalable vector check, and still
using the now deprecated implicit cast of TypeSize to uint64_t through the
overloaded operator. This patch fixes these issues.

This brings the logic in line with the comment on the context line immediately
above the added precondition.

Add a test in Redundantstores.ll that the warning is not triggered.