GlobalISel: Fix combine duplicating atomic loads (#111730)

The sext_inreg (load) combine was not deleting the old load instruction,
and it would never be deleted if volatile or atomic.

AMDGPU/GlobalISel: Handle atomic sextload and zextload (#111721)

Atomic loads are handled differently from the DAG, and have separate opcodes
and explicit control over the extensions, like ordinary

AMDGPU/GlobalISel: Handle atomic sextload and zextload (#111721)

Atomic loads are handled differently from the DAG, and have separate opcodes
and explicit control over the extensions, like ordinary loads. Add
new patterns for these.

There's room for cleanup and improvement. d16 cases aren't handled.

Fixes #111645

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