[IR] Add `llvm.modf` intrinsic (#121948)

This adds the `llvm.modf` intrinsic, legalization, and lowering (mostly
reusing the lowering for sincos and frexp).

The `llvm.modf` intrinsic takes a floating-point value and returns both
the integral and fractional parts (as a struct).

```
declare { float, float }             @llvm.modf.f32(float  %Val)
declare { double, double }           @llvm.modf.f64(double %Val)
declare { x86_fp80, x86_fp80 }       @llvm.modf.f80(x86_fp80  %Val)
declare { fp128, fp128 }             @llvm.modf.f128(fp128 %Val)
declare { ppc_fp128, ppc_fp128 }     @llvm.modf.ppcf128(ppc_fp128  %Val)
declare { <4 x float>, <4 x float> } @llvm.modf.v4f32(<4 x float>  %Val)
```

This corresponds to the libm `modf` function but returns multiple values
in a struct (rather than take output pointers), which makes it easier to
vectorize.

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