[SelectionDAG] Correctly determine the VECREDUCE_SEQ_FMUL action

The LegalizeAction for this node should follow the logic for
`VECREDUCE_SEQ_FADD` and be determined using the vector operand's type.

here isn't an in-tree target that makes use of this, but I think it's safe to
say this is how it should behave, should a target want to customize the action
for this node.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D107478