Commits on Jul 17, 2020

1. Propagate iteration info to optimizer …

   This supersedes #36169. Rather than re-implementing the iteration
   analysis as done there, this uses the new stmtinfo infrastrcture
   to propagate all the analysis done during inference all the way
   to inlining. As a result, it applies not only to splats of
   singletons, but also to splats of any other short iterable
   that inference can analyze. E.g.:
   
   ```
   f(x) = (x...,)
   @code_typed f(1=>2)
   @benchmark f(1=>2)
   ```
   
   Before:
   ```
   julia> @code_typed f(1=>2)
   CodeInfo(
   1 ─ %1 = Core._apply_iterate(Base.iterate, Core.tuple, x)::Tuple{Int64,Int64}
   └──      return %1
   ) => Tuple{Int64,Int64}
   
   julia> @benchmark f(1=>2)
   BenchmarkTools.Trial:
     memory estimate:  96 bytes
     allocs estimate:  3
     --------------
     minimum time:     242.659 ns (0.00% GC)
     median time:      246.904 ns (0.00% GC)
     mean time:        255.390 ns (1.08% GC)
     maximum time:     4.415 μs (93.94% GC)
     --------------
     samples:          10000
     evals/sample:     405
   ```
   
   After:
   ```
   julia> @code_typed f(1=>2)
   CodeInfo(
   1 ─ %1 = Base.getfield(x, 1)::Int64
   │   %2 = Base.getfield(x, 2)::Int64
   │   %3 = Core.tuple(%1, %2)::Tuple{Int64,Int64}
   └──      return %3
   ) => Tuple{Int64,Int64}
   
   julia> @benchmark f(1=>2)
   BenchmarkTools.Trial:
     memory estimate:  0 bytes
     allocs estimate:  0
     --------------
     minimum time:     1.701 ns (0.00% GC)
     median time:      1.925 ns (0.00% GC)
     mean time:        1.904 ns (0.00% GC)
     maximum time:     6.941 ns (0.00% GC)
     --------------
     samples:          10000
     evals/sample:     1000
   ```
   
   I also implemented the TODO, I had left in #36169 to inline
   the iterate calls themselves, which gives another 3x
   improvement over the solution in that PR:
   
   ```
   julia> @code_typed f(1)
   CodeInfo(
   1 ─ %1 = Core.tuple(x)::Tuple{Int64}
   └──      return %1
   ) => Tuple{Int64}
   
   julia> @benchmark f(1)
   BenchmarkTools.Trial:
     memory estimate:  0 bytes
     allocs estimate:  0
     --------------
     minimum time:     1.696 ns (0.00% GC)
     median time:      1.699 ns (0.00% GC)
     mean time:        1.702 ns (0.00% GC)
     maximum time:     5.389 ns (0.00% GC)
     --------------
     samples:          10000
     evals/sample:     1000
   ```
   
   Fixes #36087
   Fixes #29114

   

   Keno committed Jul 17, 2020
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