slowdown in regex replacement

[stevengj]

This regex benchmark from #40484 (comment):

using BenchmarkTools
s = "elephant giraffe antelope lion cheetah, "^1000;
const reg = r"elephant|giraffe|antelope|lion|cheetah"
const rep = Dict("elephant"=>"grey", "giraffe"=>"yellow", "antelope"=>"brown", "lion"=>"gold", "cheetah"=>"yellow")
@btime replace($s, $(reg=>(s->rep[s])));

on Julia 1.8.2 gives (on my x86_64 laptop):

  649.743 μs (4 allocations: 47.19 KiB)

but on master (1.10.0-DEV.551) gives:

  790.071 μs (4 allocations: 47.19 KiB)

a 20% slowdown.

The other benchmark @btime replace($s, "elephant"=>"grey", "giraffe"=>"yellow", "antelope"=>"brown", "lion"=>"gold", "cheetah"=>"yellow"); in that issue is actually 10% faster on master, so it's not all bad news.

cc @vtjnash