hir: lift alternations' common suffixes too #2538

Job	Run time
msrv	1m 26s
testfull-regex	3m 17s
test (stable, ubuntu-latest, stable)	3m 17s
miri-regex-automata	13m 53s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	3m 6s
rustfmt	4s
testfull-regex-automata	8m 18s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 54s
testfull-regex-capi	19s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	9m 33s
testfull-regex-syntax	1m 52s
test (beta, ubuntu-latest, beta)	3m 33s
test (nightly, ubuntu-latest, nightly)	2m 46s
test (macos, macos-latest, stable)	2m 20s
test (win-msvc, windows-latest, stable)	6m 45s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 21s
	1h 11m 44s

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