[llvm][SVE] Remove redundant and when comparing against extending load

When determining if an `and` should be merged into an extending load
the constant argument to the `and` is currently not checked if the
argument requires truncation. This prevents the combine happening when
the vector width is half the normal available vector width for SVE VLA
vectors.

Reviewed By: c-rhodes

Differential Revision: https://reviews.llvm.org/D129281