microsoft/compiler: Fix tautological comparison

Clang detects that a signed character can't be >= 128. Instead,
we should just explicitly check for the 8th bit via bitmask compare.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8248>

src/microsoft/compiler/dxil_module.c

@@ -185,7 +185,7 @@ static bool
 is_char7_string(const char *str)
 {
    while (*str != '\0') {
-      if (*str++ >= 128)
+      if (*str++ & 0x80)
          return false;
    }
    return true;