eval_const_expressions sometimes produced RelabelType nodes that
were useless because they just relabeled an expression to the same
exposed type it already had.  This is worth avoiding because it can
cause two equivalent expressions to not be equal(), preventing
recognition of useful optimizations.  In the test case added here,
an unpatched planner fails to notice that the "sqli = constant" clause
renders a sort step unnecessary, because one code path produces an
extra RelabelType and another doesn't.

Fix by ensuring that eval_const_expressions_mutator's T_RelabelType
case will not add in an unnecessary RelabelType.  Also save some
code by sharing a subroutine with the effectively-equivalent cases
for CollateExpr and CoerceToDomain.  (CollateExpr had no bug, and
I think that the case couldn't arise with CoerceToDomain, but
it seems prudent to do the same check for all three cases.)

Back-patch to v12.  In principle this has been wrong all along,
but I haven't seen a case where it causes visible misbehavior
before v12, so refrain from changing stable branches unnecessarily.

Per investigation of a report from Eric Gillum.

Discussion: https://postgr.es/m/CAMmjdmvAZsUEskHYj=KT9sTukVVCiCSoe_PBKOXsncFeAUDPCQ@mail.gmail.com