clang: Fix handling of __builtin_elementwise_copysign

I realized the handling of copysign made no sense at all.
Only the type of the first operand should really matter, and
it should not perform a conversion between them.

Also fixes misleading errors and producing broken IR for
integers.

We could accept different FP types for the sign argument,
if the intrinsic permitted different types like the DAG node.
As it is we would need to insert a cast, which could have
other effects (like trapping on snan) which should not happen
for a copysign.