AMDGPU: Correct maximum possible private allocation size

We were assuming a much larger possible per-wave visible stack
allocation than is possible:

https://github.com/RadeonOpenCompute/ROCR-Runtime/blob/faa3ae51388517353afcdaf9c16621f879ef0a59/src/core/runtime/amd_gpu_agent.cpp#L70

Based on this, we can assume the high 15 bits of a frame index or sret
are 0. The frame index value is the per-lane offset, so the maximum
frame index value is MAX_WAVE_SCRATCH / wavesize.

Remove the corresponding subtarget feature and option that made
this configurable.

llvm-svn: 361541