[SLP] Support FAdd/FSub as interchangeable instructions
fadd(x, c) and fsub(x, c) are interchangeable via fadd(x, -c), so treat
them like the existing Add/Sub/Mul/Shl/AShr/And/Or/Xor interchange in
BinOpSameOpcodeHelper. This lets a mixed fadd/fsub bundle vectorize as
a single op instead of needing an alt-shuffle/split node.
Restrict the interchange to a constant RHS. A constant LHS (e.g.
"0.0 - x") cannot be moved to the other opcode without also swapping
the variable operand, which would misalign it against lanes that keep
their native opcode and produce a node too expensive to vectorize.
Reviewers: hiraditya, bababuck, RKSimon
Pull Request: https://github.com/llvm/llvm-project/pull/208002
