pgsql: Use pg_stat_get_xact* functions within xacts

Use pg_stat_get_xact* functions within xacts

Resolve build farm failures from c203d6cf81b4d7e43,
diagnosed by Tom Lane.

The output of pg_stat_get_xact_tuples_hot_updated() and friends
is not guaranteed to show anything after the transaction completes.
Data is flushed slowly to stats collector, so using them can
give timing issues.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5b0d7f6996abfc1e3e51bac62af6076903635dc8

Modified Files
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src/test/regress/expected/func_index.out | 9 ++++++---
src/test/regress/sql/func_index.sql | 9 ++++++---
2 files changed, 12 insertions(+), 6 deletions(-)