[SDAG] avoid libcalls to fmin/fmax for soft-float targets

This is an extension of D70965 to avoid creating a mathlib
call where it did not exist in the original source. Also see
D70852 for discussion about an alternative proposal that was
abandoned.

In the motivating bug report:
https://github.com/llvm/llvm-project/issues/54554
...we also have a more general issue about handling "no-builtin" options.

Differential Revision: https://reviews.llvm.org/D122610