Add target hook to prevent folding some bitcasted loads.

This is to avoid this transformation in some cases:
fold (conv (load x)) -> (load (conv*)x)

On architectures that don't natively support some vector
loads efficiently casting the load to a smaller vector of
larger types and loading is more efficient.

Patch by Micah Villmow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194783 91177308-0d34-0410-b5e6-96231b3b80d8