[AMDGPU] Do not check max-bb for a single block callee

-amdgpu-inline-max-bb option could lead to a suboptimal
codegen preventing inlining of really simple functions
including pure wrapper calls. Relax the cutoff by allowing
to call a function with a single block on the grounds
that it will not increase total number of blocks after
inlining.

Differential Revision: https://reviews.llvm.org/D97744