pgsql: Fix infinite-loop risk in fixempties() stage of regex compilatio

Fix infinite-loop risk in fixempties() stage of regex compilation.

The previous coding of this function could get into situations where it
would never terminate, because successive passes would re-add EMPTY arcs
that had been removed by the previous pass. Rewrite the function
completely using a new algorithm that is guaranteed to terminate, and
also seems to be usually faster than the old one. Per Tcl bugs 3604074
and 3606683.

Tom Lane and Don Porter

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a7b61d4f5af37344f8973b2dfce47e2ba2680061

Modified Files
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src/backend/regex/regc_nfa.c | 314 +++++++++++++++++++++++++++--------
src/backend/regex/regcomp.c | 12 +-
src/test/regress/expected/regex.out | 20 +++
src/test/regress/sql/regex.sql | 7 +
4 files changed, 280 insertions(+), 73 deletions(-)