Fix a crash on function-static variables with initializers (#703)

This code path hadn't been used, and it had a crash due to not inserting the basic blocks it created (for initializing the variable) into the parent function. The fix adds a bit more smarts to the `IRBuilder` to help with inserting basic blocks into the flow of a function.

The actual user issue was around `static const` declarations, and it is clear that the code is incorrectly treating a function local `static const` as if it were just `static`. That will need to be fixed in another change.

1 files changed, 25 insertions, 0 deletions

diff --git a/tests/bugs/ir-null-parent-crash.slang b/tests/bugs/ir-null-parent-crash.slang
new file mode 100644
index 000000000..ca4d0877d
--- /dev/null
+++ b/tests/bugs/ir-null-parent-crash.slang
@@ -0,0 +1,25 @@
+// ir-null-parent-crash.slang
+
+// Test an issue where Slang was crashing on functions that
+// have `static` variables with initializers.
+
+//TEST:SIMPLE:-target hlsl
+
+struct RNG
+{
+	uint state[2];
+}
+
+void jump(inout RNG rng)
+{
+    static uint32_t a[] = { 0x4, 0x2, 0x1, 0x3 };
+
+    uint32_t s0 = 0;
+
+    for (int i = 0; i < 4; i++)
+    {
+        s0 ^= rng.state[0];
+    }
+
+    rng.state[0] = s0;
+}