* Allow hoisting `vconst` instructions out of loops
Staring at some SIMD code and what LLVM and v8 both generate it appears
that a common technique for SIMD-loops is to hoist constants outside of
loops since they're nontrivial to rematerialize unlike integer
constants. This commit updates the `loop_hoist_level` calculation with
egraphs to have a nonzero default for instructions that have no
arguments (e.g. consts) which enables hoisting these instructions out of
loops.
Note, though, that for now I've listed the maximum as hoisting outside
of one loop, but not all of them. While theoretically vconsts could move
up to the top of the function I'd be worried about their impact on
register pressure and having to save/restore around calls or similar, so
hopefully if the hot part of a program is a single loop then hoisting
out of one loop is a reasonable-enough heuristic for now.
Locally on x64 with a benchmark that just encodes binary to hex this saw
a 15% performance improvement taking hex encoding from ~6G/s to ~6.7G/s.
* Test vconst is only hoisted one loop out
18ee645ebe