Inspired by discussion on D70157. Please direct all discussion there until direction is agreed on. This isn't worthy of review until we settle on direction and syntax, it's just posted for context.
In particular, this patch is: a) using unsettled syntax which will likely change, b) has no actual implementation, and c) is missing a whole bunch of tests.
The syntax used in this patch is:
.push_align_branch_boundary 5 jcc, fused, call callq bar .pop_align_branch_boundary