[AMDGPU] Add inreg support for SGPR arguments (#67182)

Function parameters marked with inreg are supposed to be allocated to
SGPRs. However, for compute functions, this is ignored and function
parameters are allocated to VGPRs. This fix modifies CC_AMDGPU_Func in
AMDGPUCallingConv.td to use SGPRs if input arg is marked inreg.
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Co-authored-by: Jun Wang <jun.wang7@amd.com>