One xform performed by LegalizeDAG is transformation of "store of fp" to "store of int".

Make two changes:
1) only xform "store of f32" if i32 is a legal type for the target.
2) only xform "store of f64" if either i64 or i32 are legal for the target.
3) if i64 isn't legal, manually lower to 2 stores of i32 instead of letting a
   later pass of legalize do it.  This is ugly, but helps future changes I'm 
   about to commit.

llvm-svn: 42980