AtomicExpand: Fix expanding atomics into unconstrained FP in strictfp functions

Ideally the normal fadd/fmin/fmax this was creating would fail the verifier.
It's probably also necessary to force off FP exception handlers in the cmpxchg
loop but we don't have a generic way to do that now.

Note strictfp builder is broken in the minnum/maxnum case

https://reviews.llvm.org/D154993