Extend (truncate (load)) folding

DAGCombiner could fold (truncate (load)) -> smaller load if the original
load was the width of the truncation result or wider.  This patch extends
it to handle cases where the original load was narrower (and so the
extension type stays the same).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197030 91177308-0d34-0410-b5e6-96231b3b80d8