AMDGPU: More bits of frame index are known to be zero
The maximum private allocation for the whole GPU is 4G,
so the maximum possible index for a single workitem is the
maximum size divided by the smallest granularity for a dispatch.
This increases the number of known zero high bits, which
enables more offset folding. The maximum private size per
workitem with this is 128M but may be smaller still.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262153
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