R600/SI: Prefer selecting more e64 instruction forms.

Add some more tests to make sure better operand
choices are still made. Leave some cases that seem
to have no reason to ever be e64 alone.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217789 91177308-0d34-0410-b5e6-96231b3b80d8