Fix an invalid transformation by the optimizer. Introduced by #2811.

test/val/bug2937.c

@@ -0,0 +1,27 @@
+/* Note: This test is rather fragile since it depends on how the compiler
+** handles certain sequences and return codes. It checks for exactly one
+** optimizer problem introduced in #2937. If it fails in some future version,
+** do not hesitate to remove it.
+*/
+
+#include <stdio.h>
+
+static unsigned char test(unsigned v)
+{
+    (void)v;
+    asm("jsr popax");
+    asm("lda #0");
+    return __A__;
+}
+
+int main(void)
+{
+    if (test(1)) {
+        printf("Test failed\n");
+        return 1;
+    } else {
+        printf("Test ok\n");
+        return 0;
+    }
+}
+