ssa: implement -B bounds-check disabling

[cpunion]

Summary

• accept the gc-compatible llgo tool compile -B flag and propagate it through build configuration into SSA lowering
• omit index, string/slice, two/three-index slice, and slice-to-array conversion bounds checks when enabled
• retain integer-width conversions, required nil checks for *array, and mandatory unsafe.String / unsafe.Slice validity checks
• include the setting in build fingerprints and keep the default checked path unchanged
• retire the Go 1.26 fixedbugs/issue48092.go compile xfail

Scope

This change implements the compiler-action -B used by GOROOT tests and an explicit build Config setting. It does not reinterpret top-level llgo build/run/test -gcflags=-B: Go gcflags are package-pattern scoped with last-match replacement semantics, which require a separate per-package design.

The compatibility command accepts -o, but its compile-only path validates lowering without emitting an object file. This PR preserves that behavior; output-file compatibility is outside its scope.

Validation

Rebased on current xgo-dev/llgo main (73fa30fbf):

• macOS arm64, Go 1.26.5:
  • complete cmd/internal/compile, ssa, and internal/build suites pass
  • coverage: 97.4%, 87.9%, and 73.4%; combined 80.8%
  • GOROOT fixedbugs/issue48092.go passes with its xfail removed
• Linux amd64, LLVM 19, Go 1.26.5:
  • complete cmd/internal/compile, ssa, and internal/build suites pass
  • coverage: 97.4%, 88.0%, and 72.6%
  • GOROOT fixedbugs/issue48092.go passes
• key lowering coverage: checkIndex 100%, Slice 98.6%, SliceToArrayPointer 100%, and DisableBoundsChecks 100%
• checked and unchecked programs retain identical legal results; IR tests prove bounds helpers disappear under -B
• negative tests retain required *array nil panics and unsafe.String / unsafe.Slice validity panics
• git diff --check passes

The previous Ubuntu LLGo failure was unrelated to -B: it exercised the former main behavior that rejected DWARF omission for c-shared/c-archive. That support is present in current main and the rebased CI reruns it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[xushiwei]

@gemini-code-assist /review