use of illegal integer types: instead, use a stack slot
and copying via integer registers.  The existing code
is still used if the bitconvert is to a legal integer
type.

This fires on the PPC testcases 2007-09-08-unaligned.ll
and vec_misaligned.ll.  It looks like equivalent code
is generated with these changes, just permuted, but
it's hard to tell.

With these changes, nothing in LegalizeDAG produces
illegal integer types anymore.  This is a prerequisite
for removing the LegalizeDAG type legalization code.

While there I noticed that the existing code doesn't
handle trunc store of f64 to f32: it turns this into
an i64 store, which represents a 4 byte stack smash.
I added a FIXME about this.  Hopefully someone more
motivated than I am will take care of it.

llvm-svn: 60964