Test expressions like `unsigned char x = ...; ... = x / 2;`
These use `int` constants with values representable by
`unsigned int` / `unsigned char`, so using unsigned codegen should
be possible.
Additional tests for #1308. These are things we want to generate better
code for, so add tests that the behavior doesn't change.
In g_typeadjust, before we apply the integral promotions, we check if
both types are unsigned char. If so, we promote to unsigned int, rather
than int, which would be chosen by the standard rules. This is only a
performance optimization and does not affect correctness, as the flags
returned by g_typeadjust are only used for code generation, and not to
determine types of other expressions containing this one. All unsigned
char bit-patterns are valid as both int and unsigned int and represent
the same value, so either signed or unsigned int operations can be used.
This special case part is not duplicated by ArithmeticConvert.
Partial fix for #1308.
Both signed and unsigned chars are promoted to int by C's evaluation
rules. It is more efficient to use unsigned operations when possible,
however. These tests will help test the correctness of optimizations
doing that. See #1308.
