- Don't subtract thunkSize from branchRange. Most places care about the actual maximal branch range. Subtract thunkSize in the one place that wants to leave room for a thunk.
- Set it to 0x800_0000 instead of 0xFF_FFFF
- Subtract 4 for the positive branch direction since it's a two's complement 24bit number sign-extended mutiplied by 4, so its range is -0x800_0000..+0x7FF_FFFC
- Make boundary checks include the boundary values

This doesn't make a huge difference in practice. It's preparation

for a "real" fix for PR51578 -- but it also lets the repro in comment 0

in that bug place one more thunk before hitting the TODO.