[libclc] [libclc] Produce subnormal results for exp/exp2 on FP32 (PR #212696)

[via cfe-commits]

Lurie97 wrote:

@arsenm  Reworked to scale with `__clc_ldexp` directly, matching the ocml
     `expF_base.h` flow, and dropped the fixup path.

     - **Normal results unchanged:** `ldexp(y, p)` is bit-identical to the old
       `as_int(y) + (p << 23)` for normal results; only the subnormal range changes.
     - **`llim`:** `exp` uses `ln(2^-149)`, `exp2` uses `-149` — the smallest
       subnormal, rounded so the boundary input isn't flushed (no off-by-one).
     - **Clamp `[-151, 129]`:** representable `p ∈ [-149, 128]` for both, so the
       clamp only catches inputs already saturated to inf/zero by the ulim/llim
       selects. Without it, extreme inputs made `ldexp` return NaN.

     Verified on i.MX95 Mali-G310 (FTZ device): CTS `math_brute_force` exp/exp2 pass
     6/6. Re-requested your review — happy to make the clamp bounds named constants.

https://github.com/llvm/llvm-project/pull/212696