DAG: Handle expanding strict_fsub into fneg and strict_fadd

The AMDGPU handling of f16 vectors is terrible still since it gets
scalarized even when the vector operation is legal.

The code is is essentially duplicated between the non-strict and
strict case. Apparently no other expansions are currently trying to do
this. This is mostly because I found the behavior of
getStrictFPOperationAction to be confusing. In the ARM case, it would
expand strict_fsub even though it shouldn't due to the later check. At
that point, the logic required to check for legality was more complex
than just duplicating the 2 instruction expansion.