[docs] Break long (>80) line. NFC

[kristof.beyls]

LGTM

[fpetrogalli]

I forgot to add the link to the differential review in the commit message. Adding the revision here:

Committed revision 372326.
Committed 366000a to svn.

llvm/docs/Frontend/PerformanceTips.rst

 IR Best Practices
 =================
 
 As with any optimizer, LLVM has its strengths and weaknesses.  In some cases, 
 surprisingly small changes in the source IR can have a large effect on the 
 generated code.  
 
 Beyond the specific items on the list below, it's worth noting that the most 
-mature frontend for LLVM is Clang.  As a result, the further your IR gets from what Clang might emit, the less likely it is to be effectively optimized.  It 
+mature frontend for LLVM is Clang.  As a result, the further your IR gets from
+what Clang might emit, the less likely it is to be effectively optimized. It
 can often be useful to write a quick C program with the semantics you're trying
 to model and see what decisions Clang's IRGen makes about what IR to emit.  
 Studying Clang's CodeGen directory can also be a good source of ideas.  Note 
 that Clang and LLVM are explicitly version locked so you'll need to make sure 
 you're using a Clang built from the same svn revision or release as the LLVM 
 library you're using.  As always, it's *strongly* recommended that you track 
 tip of tree development, particularly during bring up of a new project.