Narrow blanket SPIR-V loop unroll in optimizer recipes

[AnastaZIuk]

Summary

• narrow legalization-time full loop unroll behind an explicit opt-in overload
• narrow legalization-time SSA rewrite behind an explicit opt-in overload
• stop materializing blanket full loop unroll in the default performance recipe
• replace the two heavy global redundancy elimination passes in the default performance recipe with local redundancy elimination
• remove several additional blanket cleanup steps from the legalization tail when the generic path does not need them
• use the DXC trunk Godbolt reproducer, which is the preprocessed output of our path tracer at about 58k LoC

Root cause

The current SPIR-V optimizer recipes still carry two old blanket unroll decisions:

• 9fbcce4ca17d added full loop unroll to legalization passes on 2018-09-19
• 3c47dac28208 added full loop unroll to performance passes on 2020-05-20

On a large preprocessed HLSL payload with many small [unroll] loops this inflates the SPIR-V module far more than necessary and then pays for expensive cleanup over that self-inflated IR.

LoopControl::Unroll as an IR hint is not the problem. The expensive part is treating that hint as a blanket request to immediately materialize full unroll in the generic optimizer path even when legality does not require it.

A similar issue existed in the legalization tail. Some cleanup passes were effectively historical safety hammers rather than semantically required defaults. Narrowing them keeps the generic path correct while removing a large amount of unnecessary work.

DXC has the producer-side lowering context and knows when a specific HLSL pattern still requires materialized loop unroll or legalize-time SSA rewrite for correctness. The companion DXC patch in microsoft/DirectXShaderCompiler#8283 supplies that narrower signal, and its current branch head also materializes the companion SPIR-V submodule pointers.

Validation

• reproducer: godbolt.org/z/o5xf1hq36 (note: Compiler Explorer cache can make repeated runs look much faster than a cold compile)
• shader payload: preprocessed output of our path tracer at about 58k LoC
• local machine: AMD Ryzen 5 5600G with Radeon Graphics, 6 physical cores, 12 logical processors, Windows-reported max clock 3901 MHz
• on the same payload and the same machine, SPIRV-Tools@487ff843bd8a + DXC@bd9a8b1c5365 reduced the workload from 19.161 s to 6.042 s
• with SPIRV-Tools@57007cf46bb4 + DXC@b02b772e0b50, the same payload measured 4.702 s
• with the current branch pair SPIRV-Tools@7134be5024ff + DXC@55112e338fd2, the same payload now measures 2.464 s
• full local CodeGenSPIRV lit/FileCheck passes with the companion DXC branch: 1403 expected passes, 2 expected failures, 0 unexpected

Companion DXC PR:
microsoft/DirectXShaderCompiler#8283