[X86] Add fadd/fsub/fmul tests showing failure to concat operands together and...

[X86] Add fadd/fsub/fmul tests showing failure to concat operands together and perform as a wider vector

We don't want to concat fadd/fsub/fmul if both operands would need concatenating (as the fp op is usually cheaper than the concat), but if at least one operand is free to concat (i.e. constant or extracted from a wider vector), then we should try to concat the fp op.