[X86] Vector 8-bit `icmp ugt + blend` with constant should use saturation arithmetic to avoid compare

Due to gaps in support on x86, unsigned vector compares are implemented by checking if one of the operands is equal to the maximum/minimum (e.g. `a >= b` => `max(a, b) == a`). This method is often paired with `blendv`, which performs a conditional selection:

```asm
selectIfGreater:
movdqa xmm3, xmm0
movdqa xmm0, xmmword ptr [rip + .LCPI0]
pminub xmm0, xmm2
pcmpeqb xmm0, xmm2
pblendvb xmm3, xmm1, xmm0
movdqa xmm0, xmm3
ret
```

https://godbolt.org/z/4Gvoqrj1P

Blend only checks the most significant bits of the "mask" input, so it is possible to use a single unsigned saturation add/sub to emulate a compare (which is one instruction shorter). The method differs slightly based on the size of the compare constant:

```
(C < 127): blendv(a, b, addSat(x, 127 - C))
(C > 127): blendv(a, b, subSat(x, C - 127))
```

This appears to only be beneficial for 8-bits due to it supporting both a granular blendv and saturation arithmetic. (Although 64-bit can benefit from a slightly modified version: #181454)

The issue names no repository file or test; start by reproducing the Godbolt example and tracing the x86 lowering for the 8-bit vector comparison and blend. Done means the constant cases use the saturation-arithmetic form where applicable and generated code is shorter, with regression coverage placed alongside the existing lowering tests.

Written by the indexing model from the issue text.