Zir: handle ranges in getMultiProng

Closes ziglang#12890

src/Zir.zig

@@ -3051,11 +3051,16 @@ pub const Inst = struct {
             var multi_i: u32 = 0;
             while (true) : (multi_i += 1) {
                 const items_len = zir.extra[extra_index];
-                extra_index += 2;
+                extra_index += 1;
+                const ranges_len = zir.extra[extra_index];
+                extra_index += 1;
                 const body_len = @truncate(u31, zir.extra[extra_index]);
                 extra_index += 1;
                 const items = zir.refSlice(extra_index, items_len);
                 extra_index += items_len;
+                // Each range has a start and an end.
+                extra_index += 2 * ranges_len;
+
                 const body = zir.extra[extra_index..][0..body_len];
                 extra_index += body_len;
 

test/behavior.zig

@@ -95,6 +95,7 @@ test {
     _ = @import("behavior/bugs/12801-1.zig");
     _ = @import("behavior/bugs/12801-2.zig");
     _ = @import("behavior/bugs/12885.zig");
+    _ = @import("behavior/bugs/12890.zig");
     _ = @import("behavior/bugs/12911.zig");
     _ = @import("behavior/bugs/12928.zig");
     _ = @import("behavior/bugs/12945.zig");

test/behavior/bugs/12890.zig

@@ -0,0 +1,18 @@
+const expect = @import("std").testing.expect;
+const builtin = @import("builtin");
+
+fn a(b: []u3, c: u3) void {
+    switch (c) {
+        0...1 => b[c] = c,
+        2...3 => b[c] = c,
+        4...7 => |d| b[d] = c,
+    }
+}
+test {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+
+    var arr: [8]u3 = undefined;
+    a(&arr, 5);
+    try expect(arr[5] == 5);
+}