[AMDGPU] Max. WG size-induced occupancy limits max. waves/EU (#137807)

The default maximum waves/EU returned by the family of
`AMDGPUSubtarget::getWavesPerEU` is currently the maximum number of
waves/EU supported by the subtarget (only a valid occupancy range in
"amdgpu-waves-per-eu" may lower that maximum). This ignores maximum
achievable occupancy imposed by flat workgroup size and LDS usage,
resulting in situations where `AMDGPUSubtarget::getWavesPerEU` produces
a maximum higher than the one from
`AMDGPUSubtarget::getOccupancyWithWorkGroupSizes`.

This limits the waves/EU range's maximum to the maximum achievable
occupancy derived from flat workgroup sizes and LDS usage. This only has
an impact on functions which restrict flat workgroup size with
"amdgpu-flat-work-group-size", since the default range of flat workgroup
sizes achieves the maximum number of waves/EU supported by the
subtarget.

Improvements to the handling of "amdgpu-waves-per-eu" are left for a
follow up PR (e.g., I think the attribute should be able to lower the
full range of waves/EU produced by these methods).

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