Big, big oops: we must flush *before* we form the argstring, otherwis…

…e the struct spreading in the calls will cause multiple eval of calls in unflushed values.

This issue cost me 16 hours of debugging.

src/backend/c.nt

@@ -734,8 +734,9 @@ class CBackendFunction : BackendFunction {
                 }
                 (:callInstr, BackendType retType, string name, int[] args): {
                     auto fpType = mod.globals.type(name).instanceOf(BackendFunctionPointerType)? else die;
-                    auto argstr = createArgString(fpType, args);
+                    // Note: this *must* come first to ensure that the argstring doesn't double eval struct values.
                     flushRegs;
+                    auto argstr = createArgString(fpType, args);
                     if (retType.size == 0) {
                         return Value("", name ~ "(" ~ argstr ~ ")", exactType=true);
                     }
@@ -748,9 +749,10 @@ class CBackendFunction : BackendFunction {
                 (:callFuncPtr, BackendType type, int regId, int[] args): {
                     auto backendFpType = type.instanceOf(BackendFunctionPointerType)? else die;
                     auto retType = backendFpType.ret;
+                    // Note: this *must* come first to ensure that the argstring doesn't double eval struct values.
+                    flushRegs;
                     auto argstr = createArgString(backendFpType, args);
                     string target = reg(regId);
-                    flushRegs;
                     if (retType.size == 0) {
                         return Value("", "(" ~ target ~ ")(" ~ argstr ~ ")", exactType=true);
                     }