[RFC] IR: Support atomicrmw FP ops with vector types (#86796)

Allow using atomicrmw fadd, fsub, fmin, and fmax with vectors of
floating-point type. AMDGPU supports atomic fadd for <2 x half> and <2 x
bfloat> on some targets and address spaces.

Note this only supports the proper floating-point operations; float
vector typed xchg is still not supported. cmpxchg still only supports
integers, so this inserts bitcasts for the loop expansion.

I have support for fp vector typed xchg, and vector of int/ptr
separately implemented but I don't have an immediate need for those
beyond feature consistency.