Add a sanity-check to tablegen to catch the case where isSimpleLoad

Add a sanity-check to tablegen to catch the case where isSimpleLoad
is set but mayLoad is not set. Fix all the problems this turned up.

Change code to not use isSimpleLoad instead of mayLoad unless it
really wants isSimpleLoad.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60459 91177308-0d34-0410-b5e6-96231b3b80d8