Reset context at the tail end of JITed EEOP_AGG_PLAIN_TRANS.

While no negative consequences are currently known, it's clearly wrong
to not reset the context in one of the branches.

Reported-By: Dmitry Dolgov
Author: Dmitry Dolgov
Discussion: https://postgr.es/m/CAGPqQf165-=+Drw3Voim7M5EjHT1zwPF9BQRjLFQzCzYnNZEiQ@mail.gmail.com
Backpatch: 11-, where JIT compilation support was added

Author: anarazel

src/backend/jit/llvm/llvmjit_expr.c

@@ -2499,6 +2499,8 @@ llvm_compile_expr(ExprState *state)
 						/* store trans value */
 						LLVMBuildStore(b, v_newval, v_transvaluep);
 						LLVMBuildStore(b, v_fcinfo_isnull, v_transnullp);
+
+						l_mcxt_switch(mod, b, v_oldcontext);
 						LLVMBuildBr(b, opblocks[i + 1]);
 
 						/* returned datum passed datum, no need to reparent */