Add @aykevl's test case for D78439

I've verified that the test passes under avr-gcc, failed in LLVM before
D78439, and passes in LLVM after D78439.

tests/unit/bugfix/misc-D78439-fix-zext-add-miscompilation.c

@@ -0,0 +1,52 @@
+// RUN: @cxx @cxxflags -mmcu=atmega328p @file -o @tempfile -O0 && avr-sim @tempfile -w TEST_STATE=u32
+
+// This integration test validates AVR compilation of a zero extended addition.
+//
+// Before D78439, the add function would incorrectly compute '393216', which should've
+// actually been '524288'.
+//
+// Ayke(@aykevl) says it best in a comment on D78439:
+//
+//     Adding 0x7ffff and 1 should result in 0x80000. However, without this patch it results in 0x60000.
+//
+//     Assembly without this patch:
+//
+//     d20:       64 0f           add     r22, r20
+//     d22:       75 1f           adc     r23, r21
+//     d24:       80 40           sbci    r24, 0x00       ; 0
+//     d26:       90 40           sbci    r25, 0x00       ; 0
+//     d28:       08 95           ret
+//
+//     With this patch:
+//
+//     d1c:       20 e0           ldi     r18, 0x00       ; 0
+//     d1e:       30 e0           ldi     r19, 0x00       ; 0
+//     d20:       64 0f           add     r22, r20
+//     d22:       75 1f           adc     r23, r21
+//     d24:       82 1f           adc     r24, r18
+//     d26:       93 1f           adc     r25, r19
+//     d28:       08 95           ret
+//
+//     The sbci not only subtracts an immediate (in this case zero), but it also subtracts the carry bit.
+//     Therefore it definitely does have an effect. This also is true for the last two adc instructions: they
+//     are needed to add the carry bit in case the previous add instruction caused a wraparound. However sbci
+//     and adc use the carry in the opposite direction (subtracting or adding it to their result).
+
+
+#include <avrlit/boilerplate/unit_test.h>
+
+uint32_t TEST_STATE = 0x1;
+
+// in one file
+__attribute__ ((noinline)) uint32_t add(uint32_t a, uint16_t b) {
+    return a + b;
+}
+
+// in another file
+__attribute__ ((noinline)) uint32_t add(uint32_t a, uint16_t b);
+
+void unit_test(void) {
+  // CHECK: after_execution(TEST_STATE) = 524288
+  TEST_STATE = add(0x7ffff, 1);
+}
+