[SPIR-V] Don't lower switch statements to OpSwitch

Description
OpSwitch even as defined with SPV_KHR_maximal_reconvergence, doesn't require converging on switch cases that have fall through.

The only way to get SPIR-V to implement switch statements that converge correctly is with OpBranch instead.

Steps to Reproduce

Given the following HLSL:

RWBuffer<int> value;

[numthreads(4, 1, 1)]
void main(uint3 threadID : SV_DispatchThreadID) {
  uint sum = 0;
  switch (value[threadID.x]) {
    case 0:
      sum += WaveActiveSum(1);
    default:
      sum += WaveActiveSum(10);
      break;
  }
  value[threadID.x] = sum;
}

CE

If given the input [ 0, 0, 1, 2], the computed output should be [ 42, 42, 40, 40 ].

Actual Behavior

Even with the KHR maximal reconvergence extension the OpSwitch is not guaranteed to converge the tangles between case 0 and the default case.

However, if instead these were generated as a chain of OpBranch statements, the control flow would converge at each new OpBranch, which would result in the correct tangle grouping.

Environment

• DXC version: All
• Host Operating System: All

Start by compiling the provided HLSL example and inspecting its generated SPIR-V, focusing on how the switch is represented. Trace the compiler's switch-lowering path and compare the current OpSwitch output with the requested OpBranch structure. Done means the example produces the expected output and switch cases converge correctly.