pgsql: expression eval, jit: Minor code cleanups.

expression eval, jit: Minor code cleanups.

This mostly consists of using C99 style for loops, moving variables
into narrower scopes, and a smattering of other minor improvements.
Done separately to make it easier to review patches with actual
functional changes.

Author: Andres Freund
Discussion: https://postgr.es/m/20191023163849.sosqbfs5yenocez3@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1ec7679f1b67e84be688a311dce234eeaa1d5de8

Modified Files
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src/backend/executor/execExpr.c       |  40 ++--
src/backend/executor/execExprInterp.c |  80 +++-----
src/backend/jit/llvm/llvmjit_expr.c   | 377 +++++++++++++++++-----------------
3 files changed, 240 insertions(+), 257 deletions(-)