AMDGPU: Fix allowing immediates for tail call pseudo.

The pseudo was using SSrc_b64, so it allowed folding immediates into
the destination operand for a tail call to null. However, this is not
a valid operand for the s_setpc_b64 this will be lowered to. Avoids
printing the operand as an invalid immediate.

Avoids a regression when tail calls are enabled in GlobalISel (somehow
tail calls to null get deleted in the DAG).