Convert the binary_coercible() and physically_coercible() functions from
SQL to plpgsql.  It's not that plpgsql is inherently better at doing
queries; if you simply convert the previous single SQL query into one
RETURN expression, it's no faster.  The problem with the existing code
is that it fools the plancache into deciding that it's worth re-planning
the query every time, since constant-folding with a concrete value for $2
allows elimination of at least one sub-SELECT.  In reality that's using the
planner to do the equivalent of a few runtime boolean tests, causing the
function to run much slower than it should.  Splitting the AND/OR logic
into separate plpgsql statements allows each if-expression to acquire a
static plan.

Also, get rid of some uses of obj_description() in favor of explicitly
joining to pg_description, allowing the joins to be optimized better.
(Someday we might improve the SQL-function-inlining logic enough that
this happens automatically, but today is not that day.)

Together, these changes reduce the runtime of the opr_sanity regression
test by about a factor of two on one of my slower machines.  They don't
seem to help as much on a fast machine, but this should at least benefit
the buildfarm.