Handle bit-field test after shift/mask

Previously, bit-field tests were incorrectly combined with load in
`if (x.bitfield)`.  Delay the test until after the shift/mask
is done.  Still combine tests with load if no shift/mask is required.

Fixes #1139

test/todo/bug1139.c

@@ -1,52 +0,0 @@
-/*
-  Copyright 2020 The cc65 Authors
-
-  This software is provided 'as-is', without any express or implied
-  warranty. In no event will the authors be held liable for any damages
-  arising from the use of this software.
-
-  Permission is granted to anyone to use this software for any purpose,
-  including commercial applications, and to alter it and redistribute it
-  freely, subject to the following restrictions:
-
-  1. The origin of this software must not be misrepresented; you must not
-     claim that you wrote the original software. If you use this software
-     in a product, an acknowledgment in the product documentation would be
-     appreciated but is not required.
-  2. Altered source versions must be plainly marked as such, and must not be
-     misrepresented as being the original software.
-  3. This notice may not be removed or altered from any source distribution.
-*/
-
-/*
-  Tests bit-field in if condition; see https://github.com/cc65/cc65/issues/1139
-*/
-
-#include <stdio.h>
-
-static unsigned char failures = 0;
-
-static struct overlap {
-    unsigned int x : 10;
-    unsigned int y : 10;
-} o = {11, 22};
-
-/* Test using bit-fields in if conditions. */
-static void test_if(void)
-{
-    o.x = 0;
-    o.y = 44;
-    if (o.x) {
-        printf("Bad, o.x is false\n");
-        failures++;
-    } else {
-        printf("Good\n");
-    }
-}
-
-int main(void)
-{
-    test_if();
-    printf("failures: %u\n", failures);
-    return failures;
-}