Implement new way of expanding extloads.

Implement new way of expanding extloads.

Now that the source and destination types can be specified,
allow doing an expansion that doesn't use an EXTLOAD of the
result type. Try to do a legal extload to an intermediate type
and extend that if possible.

This generalizes the special case custom lowering of extloads
R600 has been using to work around this problem.

This also happens to fix a bug that would incorrectly use more
aligned loads than should be used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225925 91177308-0d34-0410-b5e6-96231b3b80d8