[spv-in] Bubble up loop breaks, out of switch cases.

[eddyb]

• fixes [spv-in] Naga miscompiles SPIR-V with branches that exit more than one level of structured control flow wgpu#4449

The separate commits show the test's old (and broken) behavior, and the change induced by the fix is clear:

diff --git a/tests/out/wgsl/loop-break-from-switch.wgsl b/tests/out/wgsl/loop-break-from-switch.wgsl
index e21c3cf9..530c2ad9 100644
--- a/tests/out/wgsl/loop-break-from-switch.wgsl
+++ b/tests/out/wgsl/loop-break-from-switch.wgsl
@@ -2,17 +2,24 @@ var<private> global: i32;
 var<private> global_1: i32;
 
 fn function() {
+    var local: bool = false;
+
     let _e8 = global;
     loop {
         switch _e8 {
             case 0: {
                 global_1 = 0;
+                local = true;
                 break;
             }
             default: {
                 break;
             }
         }
+        let _e11 = local;
+        if _e11 {
+            break;
+        }
         global_1 = -9;
         break;
     }

[cwfitzgerald]

Hello, thank you for your PR against Naga!

As part of gfx-rs/wgpu#4231, we have moved development of Naga into the wgpu repository in the Naga subfolder. We have transferred all issues, but we are unable to automatically transfer PRs.

As such, please recreate your PR against the wgpu repository. We apologize for the inconvenience this causes, but will make contributing to both projects more streamlined going forward.

We are leaving PRs open, but once they are transferred, please close the original Naga PR.