Begin attempt at a more thorough register-allocation pass.
A toy prototype implementation of full liveness analysis.

It seems to work, and produce better register allocations on some small
examples.  But the runtime blows up for complex functions, so more work
is definitely needed!
A much-improved take on registerization via full liveness analysis.

This version still runs painfully slowly for large functions, but it
has a lot better memory usage and scope for improvement.
Full-liveness registerization now working well for simple functions.
Fix issue with in-block liveness detection
Trying to tweak new registerization to run a little faster.
More speedups for registerize
Reduce number of iteratins required for liveness analysis.
Doc updates for registerizeHarder
Remove extraneous edits for PR
Doc tweaks
Add preliminary special-case handling of 'label' variable.
Add some more node types to the flow-graph builder.
Fix some registerization bugs uncovered by the testsuite.
Fix bug in labelled-jump handling during registerization.
It's ok to have label=0 in the middle of a block
Update for new method of generating minified names.
Simplify registerizeHarder by assuming asmjs.
Only registerizeHarder on full-optimize asm builds.
WIP