AMDGPU: Simplify placeholder replacement in AMDGPUPromoteAlloca (#188202)
If `promoteAllocaUserToVector` returns the placeholder, it means the
instruction does not actually modify the alloca. we don't need to add
the placeholder as block available value for correctness. Instructions
appear afterwards in the the same block could still get the placeholder
as source value through GetCurVal() call. Instructions in other block
which access the alloca will be set up later when we really do
placeholder replacement.
This help simplify the placeholder replacement logic.