Bug 19348: Check for legal ExtLoad operation before folding

(aext (zextload x)) -> (aext (truncate (*extload x)))

Patch by Stanislav Mekhanoshin!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205805 91177308-0d34-0410-b5e6-96231b3b80d8