[HLSL] `[branch]` attribute support for `switch` statements

[llvm-beanz]

HLSL supports the branch attribute on switch statements. Clang currently errors if you apply the [branch] attribute to a switch statement.

RWBuffer<uint4> Buf;

[numthreads(8,1,1)]
void main(uint GI : SV_GroupIndex) {
  uint4 Val = Buf[GI];
  [branch]
  switch (Val.x) {
    case 0:
      Buf[GI] = Val.xxxx;
      break;
    case 1:
      Buf[GI] = Val.yyyy;
      break;
    case 2:
      Buf[GI] = Val.zzzz;
      break;
    default:
      Buf[GI] = Val.wwww;
      break;
  }
}

Godbolt

[damyanp]

We need to do some investigation before we attempt to estimate this.