* introduce a dump() function on Module.Fn which helpfully prints to
stderr the ZIR representation of a function (can be called before
attempting to codegen it). This is a debugging tool.
* implement x86 codegen for loops
* liveness: fix analysis of conditional branches. The logic was buggy
in a couple ways:
- it never actually saved the results into the IR instruction (fixed now)
- it incorrectly labeled operands as dying when their true death was
after the conditional branch ended (fixed now)
* zir rendering is enhanced to show liveness analysis results. this
helps when debugging liveness analysis.
* fix bug in zir rendering not numbering instructions correctly
closes#6021
