[x86] Fix PR22524: the DAG combiner was incorrectly handling illegal

nodes when folding bitcasts of constants.

We can't fold things and then check after-the-fact whether it was legal.
Once we have formed the DAG node, arbitrary other nodes may have been
collapsed to it. There is no easy way to go back. Instead, we need to
test for the specific folding cases we're interested in and ensure those
are legal first.

This could in theory make this less powerful for bitcasting from an
integer to some vector type, but AFAICT, that can't actually happen in
the SDAG so its fine. Now, we *only* whitelist specific int->fp and
fp->int bitcasts for post-legalization folding. I've added the test case
from the PR.

(Also as a note, this does not appear to be in 3.6, no backport needed)

llvm-svn: 228656