[SelectionDAG] Flags are dropped when creating a new FMUL (#66701)

While simplifying some vector operators in DAG combine, we may need to
create new instructions for simplified vectors. At that time, we need to
make sure that all the flags of the new instruction are copied/modified
from the old instruction.

If "contract" is dropped from an instruction like FMUL, it may not
generate FMA instruction which would impact performance.

Here's an example where "contract" flag is dropped when FMUL is created.

Replacing.2 t42: v2f32 = fmul contract t41, t38
With: t48: v2f32 = fmul t38, t38

Co-authored-by: Sirish Pande <sirish.pande@amd.com>