AMDGPU: Implement i1 to bfloat conversion (#130831)
We are using the same approach as the conversion of other integer type
to bfloat: i1 --> f32 and f32 --> bf16. Refer to LowerUINT_TO_FP and
LowerSINT_TO_FP in AMDGPUTargetLowering.cpp for details.
Fixes: SWDEV-511605
