Inline the easy cases in MakeExpandedObjectReadOnly().

This attempts to buy back some of whatever performance we lost from fixing
bug #14174 by inlining the initial checks in MakeExpandedObjectReadOnly()
into the callers.  We can do that in a macro without creating multiple-
evaluation hazards, so it's pretty much free notationally; and the amount
of code added to callers should be minimal as well.  (Testing a value can't
take many more instructions than passing it to a subroutine.)

Might as well inline DatumIsReadWriteExpandedObject() while we're at it.

This is an ABI break for callers, so it doesn't seem safe to put into 9.5,
but I see no reason not to do it in HEAD.