pgsql: Make parseNodeString() C idiom compatible with Visual Studio 201

Make parseNodeString() C idiom compatible with Visual Studio 2015.

Between v15 and now, this function's "else if" chain grew from 252 lines
to 592 lines, exceeding a compiler limit that manifests as "fatal error
C1026: parser stack overflow, program too complex (compiling source file
src/backend/nodes/readfuncs.c)".  Use "if (...)  return ...;" instead.

Reviewed by Tom Lane, Peter Eisentraut and Michael Paquier.  Not all
reviewers endorse this.

Discussion: https://postgr.es/m/20230607185458.GA1334487@rfd.leadboat.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/f9f31aa91f82df863a35354893978e1937863d7c

Modified Files
--------------
src/backend/nodes/gen_node_support.pl |  4 ++--
src/backend/nodes/readfuncs.c         | 12 ++----------
2 files changed, 4 insertions(+), 12 deletions(-)