Make compilation faster :rocket:

I did some investigation on the speed of compilation. This is getting important because of things like j2cl output which is a 50MB real-world file.

Slowest passes: (in -O3; note that some passes run more than once)

[PassRunner]   running pass: precompute-propagate...           14.23460 seconds.
[PassRunner]   running pass: dae...                            10.91160 seconds.
[PassRunner]   running pass: heap2local...                     10.44350 seconds.
[PassRunner]   running pass: precompute-propagate...            9.07261 seconds.
[PassRunner]   running pass: inlining...                        8.02837 seconds.
[PassRunner]   running pass: once-reduction...                  8.02484 seconds.
[PassRunner]   running pass: vacuum...                          5.16162 seconds.
[PassRunner]   running pass: vacuum...                          3.64088 seconds.
[PassRunner]   running pass: vacuum...                          3.58191 seconds.
[PassRunner]   running pass: vacuum...                          3.61897 seconds.

perf reports almost 15% of time is spent in malloc/free methods - quite a lot. I counted how many mallocs (note: not the size of them) are done in each pass, here are the heaviest (note: timings here include the overhead of counting mallocs atomically among threads, so those matter less):

    once-reduction...                 9.51466 seconds.  141558196 mallocs. 
    precompute-propagate...           14.7426 seconds.   61611533 mallocs. 
    dae...                            11.3789 seconds.   53792948 mallocs. 
    precompute-propagate...           9.69919 seconds.   39531493 mallocs. 
    heap2local...                     10.5116 seconds.   38582302 mallocs. 
    local-cse...                      4.70765 seconds.   33454468 mallocs. 
    cfp...                            2.43619 seconds.   32292682 mallocs. 
    ssa-nomerge...                    4.92099 seconds.   26378117 mallocs. 
    simplify-locals-nostructure...    6.03281 seconds.   22663794 mallocs. 
    vacuum...                         5.10813 seconds.   23880789 mallocs. 
    inlining...                       107.781 seconds.   20940365 mallocs. 
    dce...                            3.84722 seconds.   20025273 mallocs. 
    local-subtyping...                3.44374 seconds.   19067529 mallocs. 

and vacuum runs 3 more times:

    vacuum...                         3.71648 seconds.   16648663 mallocs. 
    vacuum...                         3.86917 seconds.   16302767 mallocs. 
    vacuum...                         3.89855 seconds.   16162915 mallocs. 

Help in speeding things up would be very welcome!

The issue provides -O3 timing and malloc counts for passes including once-reduction, precompute-propagate, dae, heap2local, inlining, and vacuum, but names no files or tests. Start by reproducing the measurements on the 50MB j2cl output and profiling those passes; done means a documented, measurable reduction in compilation time.