Fix SPIR-V structured control flow violation for continue in switch inside for-loop

[Copilot]

A continue statement inside a switch nested within a for loop generates invalid SPIR-V that fails validation: "block exits the selection but not via a structured exit."

for (int i = 0; i < 3; ++i) {
    switch (i) {
        case 0: continue;  // Breaks SPIR-V structured control flow
    }
}

Root Cause

Commit 412d347 added continueBlock to the exitBlocks set when processing nested constructs but never removed it. This caused the continue block to persist in the global set, affecting subsequent region processing.

Changes

• slang-ir-eliminate-multilevel-break.cpp: Added cleanup to remove continueBlock from exitBlocks after processing each region, matching the existing cleanup pattern for info.exitBlocks
• tests/bugs/for-loop-switch-continue.slang: Added regression test

The fix ensures proper SPIR-V structured control flow by preventing the continue block from leaking across region boundaries.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>For loop continue inside a switch breaks structured control flow</issue_title>
<issue_description># Issue Description
Related to #8685, but this one is a regression. The repro code was fine with v2025.21.2, but it's broken in v2025.24.3.

Reproducer Code

[shader("fragment")]
float main(): SV_Target0 {
    float sum = 0;

    for (int i = 0; i < 1; ++i) {
        sum += 1;

        switch (i) {
            case 0: {
                continue;
            }
        }
    }

    return sum;
}

slangc a.slang -target spirv -O2 -o out.spv && spirv-val --target-env vulkan1.2 out.spv
error: line 45: block <ID> '28[%28]' exits the selection headed by <ID> '25[%25]', but not via a structured exit
  %28 = OpLabel

Environment

• Slang Version 2025.24.3
• OS Linux 6.18.2-arch2-1 x86_64</issue_description>

<agent_instructions>Please reproduce this issue and then bisect git tree to find out culprit.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes For loop continue inside a switch breaks structured control flow #9585

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