RFA: Document first operand to RTX_AUTOINC

[Richard Sandiford]

As Jeff suggested here:
https://gcc.gnu.org/ml/gcc-patches/2014-08/msg00390.html
this patch documents that the first operand to an RTX_AUTOINC
is the automodified register.

Tested on x86_64-linux-gnu.  OK to install?

Thanks,
Richard


gcc/
	* doc/rtl.texi (RTX_AUTOINC): Document that the first operand is
	the automodified register.

Index: gcc/doc/rtl.texi
===================================================================
--- gcc/doc/rtl.texi	2014-05-06 18:38:47.982200623 +0100
+++ gcc/doc/rtl.texi	2014-08-30 16:04:50.870456416 +0100
@@ -193,7 +193,8 @@ An RTX code for something that matches i
 
 @item RTX_AUTOINC
 An RTX code for an auto-increment addressing mode, such as
-@code{POST_INC}.
+@code{POST_INC}.  @samp{XEXP (@var{x}, 0)} gives the auto-modified
+register.
 
 @item RTX_EXTRA
 All other RTX codes.  This category includes the remaining codes used