[Clang] Fix an unused-but-set-variable warning with volatile variable

For the following code,
    void test() {
        volatile int j = 0;
        for (int i = 0; i < 1000; i++)
                j += 1;
        return;
    }
If compiled with
    clang -g -Wall -Werror -S -emit-llvm test.c
we will see the following error:
    test.c:2:6: error: variable 'j' set but not used [-Werror,-Wunused-but-set-variable]
            volatile int j = 0;
                         ^

This is not quite right since 'j' is indeed used due to '+=' operator.
gcc doesn't emit error either in this case.
Also if we change 'j += 1' to 'j++', the warning will disappear
with latest clang.

Note that clang will issue the warning if the volatile declaration
involves only simple assignment (var = ...).

To fix the issue, in function MaybeDecrementCount(), if the
operator is a compound assignment (i.e., +=, -=, etc.) and the
variable is volatile, the count for RefsMinusAssignments will be
decremented, similar to 'j++' case.

Differential Revision: https://reviews.llvm.org/D121715