AMDGPU MIR serialization missing handling of MinNumAGPRs

[arsenm]

This field is missing MIR serialization

It needs to be added to the yaml mirror here

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Matt Arsenault (arsenm)

[This field ](https://github.com/llvm/llvm-project/blob/e3a376fb72798b5f3353f0d8b7f1b92e76a096f4/llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.h#L514) is missing MIR serialization

It needs to be added to the yaml mirror here