This avoids a vmerge at the end and avoids spurious fflags updates.
This isn't used for constrained intrinsic so we technically don't have
to worry about fflags, but it doesn't cost much to support it.
To support I've extend our FCOPYSIGN_VL node to support a passthru
operand. Similar to what was done for VRGATHER*_VL nodes.
I plan to do a similar update for trunc, floor, and ceil.
Why is the explicit splat here needed? getConstantFP seems to do generate a splat internally, and the old code relied on that behavior. Why change it to create the scalar constant (explicitly) and then splat (explicitly)?