cmd: unblock LLVM bit operations and stack layouts

[zhouguangyuan0718]

Follow-up to the merged #74. This PR now intentionally excludes the proposed StackObject live-root materialization; that design and all dependent whitelist claims have been removed.

Changes:

• lower Ctz8/16/32/64, including NonZero, through llvm.cttz
• lower PopCount8/16/32/64 through llvm.ctpop
• treat zero-length arrays of pointer-containing elements as pointer-free layouts
• qualify seven Darwin/arm64 runtime cases that pass independently of any StackObject change

Explicitly not included:

• no .gcroot materialization for live StackObject addresses
• no changes to alloca-statepoint GoObj expectations
• fixedbugs/issue27518b.go remains outside the whitelist
• llvm_alloca_statepoint_gc.go remains outside the whitelist

Validation:

• cmd/compile/internal/ssa: pass with llvm23,dynamicllvm
• GoALLC plugin CTest: 57/57
• codegen/mathbits.go: pass
• alloca-statepoint retains the existing contract: FuncInfo.locals=88, LocalsPointerMaps=[[]], stack-map queries=[0 0 0 0 0]
• cmp.go, convert4.go, ddd.go, issue16095.go, issue32288.go, nilptr2.go, and typeparam/list2.go: 3/3 each without the removed StackObject implementation

Current Darwin/arm64 whitelist size: 72 codegen, 1075 runtime.

[zhouguangyuan0718]

Final qualification after rebasing on the refreshed #74:

• full Test/LLVM: 72/72 codegen, 1077/1077 runtime, 1842.223s
• plugin CTest after an explicit ccache-backed rebuild: 57/57
• alloca-statepoint GoObj: FuncInfo.locals=104, LocalsPointerMaps=[[] [11]], stack-map queries=[1 1 1 1 0]
• issue27518b.go: 3/3 after the final rebuild
• stackobj.go, stackobj2.go, stackobj3.go: pass

The runtime.newproc caller fake-use experiment was deliberately dropped: llvm_newproc_gc.go still failed 5/5 with captured payload 0, so the remaining defect is in publication/new-goroutine funcval context rather than only caller safepoint liveness.

[zhouguangyuan0718]

StackObject correction applied: the scoped .gcroot implementation, modified alloca/GoObj expectations, issue27518b whitelist entry, and llvm_alloca_statepoint_gc whitelist entry have all been removed. The branch was rebuilt directly on merged go1.27.master. Seven remaining runtime additions were rerun 3x each without any StackObject change and passed; plugin CTest is 57/57 and the existing alloca-statepoint contract remains locals=88 / LocalsPointerMaps=[[]].