[RISCV] Fix comment in compress-opt-branch.ll to match description. NFC

Test description says constant does not fit in 12 bits, but the constant
used was -2048 which does fit in 12 bits. Update to -2049.

Also remove uses of -NOT in favor of positive checks. One of the -NOT
should have been using RESBROPT instead of "c.beqz" so that it would
check for the absense of the correct instruction based on the sed
replacement on the RUN line.