Simplify `Hir::dot` constructors #2243

Job	Run time
miri-regex-automata	10m 34s
rustfmt	14s
msrv	2m 6s
test (stable, ubuntu-latest, stable)	3m 54s
testfull-regex	3m 48s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	2m 49s
testfull-regex-automata	8m 48s
testfull-regex-capi	29s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 37s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	9m 51s
testfull-regex-syntax	2m 0s
test (beta, ubuntu-latest, beta)	3m 53s
test (nightly, ubuntu-latest, nightly)	3m 45s
test (macos, macos-latest, stable)	2m 48s
test (win-msvc, windows-latest, stable)	5m 57s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 26s
	1h 11m 59s

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