[CodeGen] Fix FCOPYSIGN legalization to account for mismatched types.

We used to legalize it like it's any other binary operations.  It's not,
because it accepts mismatched operand types.  Because of that, we used
to hit various asserts and miscompiles.

Specialize vector legalizations to, in the worst case, unroll, or, when
possible, to just legalize the operand that needs legalization.

Scalarization isn't covered, because I can't think of a target where
some but not all of the 1-element vector types are to be scalarized.

llvm-svn: 243924