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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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/1dd92380b00781b8318d6d3fb2817468e43fe3a5

Modified Files
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src/backend/regex/regc_nfa.c | 314 ++++++++++++++++++++++++++++++++---------
src/backend/regex/regcomp.c | 12 +-
2 files changed, 253 insertions(+), 73 deletions(-)