Set RTX_UNCHANGING is readonly_field_p when clearing

[Richard Kenner]

    There is nothing wrong with the manual explaining how to get the bit set
    correctly.

Done thusly, to avoid duplicating information which might change:

2003-04-21  Richard Kenner  <kenner@vlsi1.ultra.nyu.edu>
	
	* doc/rtl.texi (RTX_UNCHANGING_P): Point to true_dependence for
	details of conflict handling.
	
*** doc/rtl.texi	17 Apr 2003 01:11:21 -0000	1.51
--- doc/rtl.texi	21 Apr 2003 22:10:24 -0000
***************
*** 724,731 ****
  @cindex @code{unchanging}, in @code{reg} and @code{mem}
  @item RTX_UNCHANGING_P (@var{x})
! Nonzero in a @code{reg}, @code{mem}, or @code{concat} if the memory
! is set at most once,
! anywhere.  This does not mean that it is function invariant.
! Stored in the @code{unchanging} field and printed as @samp{/u}.
  
  @findex SCHED_GROUP_P
--- 724,733 ----
  @cindex @code{unchanging}, in @code{reg} and @code{mem}
  @item RTX_UNCHANGING_P (@var{x})
! Nonzero in a @code{reg}, @code{mem}, or @code{concat} if the register or
! memory is set at most once, anywhere.  This does not mean that it is
! function invariant.  This flag is used to determine whether two
! references conflict.  See @code{tree_dependence} in @file{alias.c} for
! more details.  Stored in the @code{unchanging} field and printed as
! @samp{/u}.
  
  @findex SCHED_GROUP_P