R600/SI: Fix selection failure on scalar_to_vector

There seem to be only 2 places that produce these,
and it's kind of tricky to hit them.

Also fixes failure to bitcast between i64 and v2f32,
although this for some reason wasn't actually broken in the
simple bitcast testcase, but did in the scalar_to_vector one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210664 91177308-0d34-0410-b5e6-96231b3b80d8