doc fix for case insensitive filesystems

[Mike Stump]

Here is a small patch to make the html output work on case insensitive
filesystems.  The problem is there is another {Loop Representation},
identical, but a different case, but meant to be different.

While this obviously fixes it, the right name for the existing {Loop
Representation} node and the {Loop representation} node isn't (to me).
Could a loop person or a doc person suggest better node names?

If not, Ok?

Doing diffs in doc:
--- doc/loop.texi.~1~   2007-01-17 09:58:59.000000000 -0800
+++ doc/loop.texi       2007-01-26 11:47:10.000000000 -0800
@@ -17,7 +17,7 @@ RTL, as well as the interfaces to loop-r
 variable analysis and number of iterations analysis).
 
 @menu
-* Loop representation::                Representation and analysis of loops.
+* Loop internal representation:: Representation and analysis of loops.
 * Loop querying::              Getting information about loops.
 * Loop manipulation::          Loop manipulation functions.
 * LCSSA::                      Loop-closed SSA form.
@@ -28,9 +28,9 @@ variable analysis and number of iteratio
 * Lambda::                     Linear loop transformations framework.
 @end menu
 
-@node Loop representation
-@section Loop representation
-@cindex Loop representation
+@node Loop internal representation
+@section Loop internal representation
+@cindex Loop internal representation
 @cindex Loop analysis
 
 This chapter describes the representation of loops in GCC, and functions
--------------