Similar to what we do for addx. Since we're calculating b - a and
because subtraction is not communitative, we can only apply this when
source register a holds the constant.
Before:
45 8B EE mov r13d,r14d
41 83 ED 08 sub r13d,8
After:
45 8D 6E F8 lea r13d,[r14-8]
19dda51a0d