pgsql: Inline plpgsql's exec_stmt() into exec_stmts().

Inline plpgsql's exec_stmt() into exec_stmts().

This saves one level of C function call per plpgsql statement executed,
and permits a tiny additional optimization of not saving and restoring
estate->err_stmt for each statement in a block.  The net effect seems
nearly un-measurable on x86_64, but there's a clear win on aarch64,
amounting to two or three percent in a loop over a few simple plpgsql
statements.

To do this, we have to get rid of the other existing call sites for
exec_stmt().  Replace them with exec_toplevel_block(), which is just
defined to do what exec_stmts() does, but for a single
PLpgSQL_stmt_block statement.  Hard-wiring the expectation of which
statement type applies here allows us to skip the dispatch switch,
making this not much uglier than the previous factorization.

Amit Khandekar, tweaked a bit by me

Discussion: https://postgr.es/m/CAJ3gD9eBNrmUD7WBBLG8ohaZ485H9y+4eihQTgr+K8Lhka3vcQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1f902d499eda862a98e881a2227b8d2cf3565374

Modified Files
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src/pl/plpgsql/src/pl_exec.c | 280 +++++++++++++++++++++++--------------------
1 file changed, 147 insertions(+), 133 deletions(-)