x86: simplify expressions in update_imm()

x86: simplify expressions in update_imm()

Comparing the sum of the relevant .imm<N> fields against a constant imo
makes more obvious what is actually meant. It allows dropping of two
static variables, with a 3rd drop requiring two more minor adjustments
elsewhere, utilizing that "i" is zeroed first thing in md_assemble().
This also increases the chances of the compiler doing the calculations
all in registers.