Reuse widening division results in integer and decimal arithmetic

[benaadams]

Reuse quotient/remainder results in wide-integer and decimal arithmetic instead of reconstructing remainders or decomposing inputs before an available widening divide:

• Route 64-bit-target Int128 DivRem and remainder through UInt128.DivRem, preserving signed overflow and remainder semantics.
• Prefer existing X86Base widening-divide intrinsics in BigIntegerCalculator when the quotient fits in one limb; reuse UInt128.DivRem in the fallback.
• Feed separate limbs to Decimal's widening division and rounding paths.
• Use full-limb division in the IEEE decimal wide power-of-ten helper.

Split from #134039 as suggested by @tannergooding. This uses existing APIs and JIT support and can merge independently of the new carry, multiply-chain and remainder-recognition optimizations.

Includes BigInteger-based boundary and randomized oracles for Int128, BigInteger limb division, decimal remainder/rounding, and the uint/ulong/UInt128 IEEE wide-division helper.

Validation:

• Rebuilt Checked x64 runtime/CoreLib and Release System.Runtime.Numerics; focused tests passed with the baseline JIT, including register stress and disabled hardware intrinsics.
• Final analyzer validation enabled IDE0005, code-style enforcement and documentation generation. No IDE0005 diagnostics in the changed files or new tests. Existing unused-import diagnostics elsewhere in CoreLib/Numerics were retained as warnings for this validation; an unrelated existing CA1018 diagnostic was excluded locally. No diagnostic suppressions are added by this PR.
• The broad shared-framework build with documentation generation hit unrelated existing documentation diagnostics; focused affected-project validation was used instead.
• git diff --check passed. No new throughput claim, native ARM64 run or x86 execution measurement is included in this extraction.

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[Copilot]

Copilot review overview

🔵 Needs a closer look

A moderate performance concern remains in the BigInteger widening-divide path and requires final human review.

Review effort: Lite
Findings: None

What changed in this PR

Optimizes wide integer and decimal division by reusing quotient/remainder results and widening-division intrinsics, with focused oracle-backed tests.

Changes:

• Adds widening-division paths for Int128, BigInteger, and decimal arithmetic.
• Reuses division results for remainder and rounding operations.
• Adds boundary and randomized regression tests.

File	Description
src/​tests/​JIT/​opt/​Add/​WideDivision.csproj	Configures wide-division tests.
src/​tests/​JIT/​opt/​Add/​WideDivision.cs	Tests integer, BigInteger, and decimal division.
src/​tests/​JIT/​opt/​Add/​DecimalWideDivision.csproj	Configures IEEE decimal tests.
src/​tests/​JIT/​opt/​Add/​DecimalWideDivision.cs	Tests wide power-of-ten division.
src/​libraries/​System.Private.CoreLib/​src/​System/​Numerics/​BigIntegerCalculator.Shared.cs	Adds widening-divide fast paths and fallback reuse.
src/​libraries/​System.Private.CoreLib/​src/​System/​Number.DecimalIeee754.cs	Adds full-limb IEEE decimal division.
src/​libraries/​System.Private.CoreLib/​src/​System/​Int128.cs	Reuses unsigned division results for signed operations.
src/​libraries/​System.Private.CoreLib/​src/​System/​Decimal.DecCalc.cs	Reuses widening division for decimal remainder and rounding.