Fixed a bug that didn't preserve the accumulator's value when a simple 16-bit fetch-and-store is optimized. (#637)

Fix a 16-bit fetch-and-store cc65 optimizer bug.

test/val/assign-use1.c

@@ -0,0 +1,36 @@
+/*
+  !!DESCRIPTION!! Assign an int; then, do an operation that depends directly on that assignment.
+  !!ORIGIN!!      cc65 regression tests
+  !!LICENCE!!     Public Domain
+  !!AUTHOR!!      Greg King
+*/
+
+#include <stdio.h>
+
+static unsigned char failures = 0;
+
+static unsigned int result;
+static const unsigned int buffer = 0xABCD;
+
+int main(void)
+{
+    result = buffer;
+
+    /* Shift doesn't use high byte (X register); previous assignment should be optimized. */
+    result <<= 8;
+    if (result != 0xCD00) {
+        ++failures;
+        printf("assign-use1: left shift is $%X, not $CD00.\n", result);
+    }
+
+    result = buffer;
+
+    /* Shift does use high byte; previous assignment shouldn't be optimized by OptStore5(). */
+    result >>= 8;
+    if (result != 0x00AB) {
+        ++failures;
+        printf("assign-use1: right shift is $%X, not $00AB.\n", result);
+    }
+
+    return failures;
+}