Fix an issue with isGuaranteedToTransferExecutionToSuccessor

I'm not sure if this was intentional, but today
isGuaranteedToTransferExecutionToSuccessor returns true for readonly and
argmemonly calls that may throw.  This commit changes the function to
not implicitly infer nounwind this way.

Even if we eventually specify readonly calls as not throwing,
isGuaranteedToTransferExecutionToSuccessor is not the best place to
infer that.  We should instead teach FunctionAttrs or some other such
pass to tag readonly functions / calls as nounwind instead.

llvm-svn: 290794