[llvm] [X86] Add rewrite pattern for SSE41/AVX1 roundss/sd + blendps/pd (PR #172056)

[Gergo Stomfai via llvm-commits]

stomfaig wrote:

@RKSimon, could you help with deciding what to do about `ROUNDSSri_Int`? The current definition:

```
let Constraints = "$src1 = $dst" in
defm ROUND  : sse41_fp_unop_s_int<0x0A, 0x0B, "round", SchedWriteFRnd.Scl,
                                  v4f32, v2f64, X86RndScales>; 
```

makes it impossible to truly combine the round and blend ops, since `src1 = dst`. This might be on purpose though, so I don't want to change it without your approval.

https://github.com/llvm/llvm-project/pull/172056