This is a useful optimization for targets with the portable garbage
collector. It isn't as big as you might guess but it does optimize
functions inside the garbage collector itself (which obviously should
not allocate). WebAssembly output in one test is about 1% smaller.
Some instructions do not create new values, they transform existing
values in some way (bitcast, getelementptr, etc.). Do not store them in
the stack object.
This lowers the size of the repulsion demo from 100kB to 98kB (with a
baseline of 72kB for the leaking GC). That's a useful reduction in file
size.
