Fix incorrect CoercedConstantValue usage in IsVNIntegralConstant

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On 32-bit targets ssize_t is effectively int32_t, so CoercedConstantValue<ssize_t> truncates TYP_LONG values. In IsVNIntegralConstant, this truncation occurs before the FitsIn<T> check, causing arbitrary TYP_LONG constants to be incorrectly reported as fitting into smaller integral types (e.g. int32).

Changes

• src/coreclr/jit/valuenum.h: read the constant as int64_t instead of ssize_t in IsVNIntegralConstant, preserving the full 64-bit value before the FitsIn<T> check.

// before
ssize_t val = CoercedConstantValue<ssize_t>(vn);
// after
int64_t val = CoercedConstantValue<int64_t>(vn);

TYP_INT constants are sign-extended to int64_t and TYP_LONG constants are preserved without truncation, so FitsIn<T> now evaluates the true value on all architectures.

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[Copilot]

Pull request overview

This PR fixes IsVNIntegralConstant so JIT value-number constants are checked for integral fit after preserving the full 64-bit value, avoiding 32-bit host truncation of TYP_LONG constants.

Changes:

• Reads VN integral constants as int64_t before applying FitsIn<T>.
• Preserves existing behavior for TYP_INT while correctly rejecting oversized TYP_LONG values.