More optimizations and a fix for is_vararg_type

[timholy]

These are all low-level optimizations. The cumulative effect is to go from ~18us/iteration in the summer benchmark to ~16us/iteration. One of the commits, the is_vararg_type, actually leads to a 3% hit on that benchmark, but it fixes what seems to be an outright mistake. (Before 22a8b2c it was not possible to step into any function that received Union{} as an argument; the case that I observed was for promote_result.)

On the summer benchmark, it seems the only remaining case of dynamic dispatch arises from JuliaLang/julia#31565.

While performance improvements are always nice, my real goal in doing this is to prepare the way for a trial of the effect of lowered-code inlining (ref. #204 (comment)). Since I am hoping it has a dramatic effect, it would be a shame if it were "spoiled" by pre-existing bottlenecks, so I thought I'd start by getting rid of them even though currently they aren't terribly important.

To develop inlining, I think the best approach would be to first take a simple case, like summer, and "inline by hand" to see if it provides as much improvement as I hope. If so, then it will be worth building out the more general machinery.

[codecov-io]

Codecov Report

Merging #254 into master will decrease coverage by 1.69%.
The diff coverage is 83.33%.

@@            Coverage Diff            @@
##           master     #254     +/-   ##
=========================================
- Coverage   87.15%   85.45%   -1.7%     
=========================================
  Files          14       14             
  Lines        1985     2049     +64     
=========================================
+ Hits         1730     1751     +21     
- Misses        255      298     +43

Impacted Files	Coverage Δ
src/interpret.jl	86.4% <ø> (ø)	⬆️
src/utils.jl	87% <100%> (ø)	⬆️
src/generate_builtins.jl	89.77% <80%> (-0.82%)	⬇️
src/builtins-julia1.0.jl	72.18% <0%> (-6.61%)	⬇️
src/builtins-julia1.1.jl	73.45% <0%> (-5.98%)	⬇️
src/builtins-julia1.2.jl	73.17% <0%> (-5.86%)	⬇️

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[KristofferC]

What is the summer benchmark?

[timholy]

Sorry, it's just

JuliaInterpreter.jl/test/interpret.jl

Lines 8 to 14 in cef5350

	function summer(A)
	s = zero(eltype(A))
	for a in A
	s += a
	end
	return s
	end

with a Float64 array of large enough size (e.g., 10^5) that initialization isn't a big deal. I usually do

frame = JuliaInterpreter.enter_call(summer, A)
@elapsed(JuliaInterpreter.finish_and_return!(frame))/length(A)