[AggressiveInstCombine][Docs] Update pass documentation

[kitaisreal]

Summary

Updated AggressiveInstCombine pass documentation because now AggressiveInstCombine can modify CFG.
Introduced in D154725.

[nikic]

LGTM

llvm/docs/Passes.rst

 library calls on different targets.
 
 .. _passes-aggressive-instcombine:
 
 ``-aggressive-instcombine``: Combine expression patterns
 --------------------------------------------------------
 
 Combine expression patterns to form expressions with fewer, simple instructions.
-This pass does not modify the CFG.
 
 For example, this pass reduce width of expressions post-dominated by TruncInst
 into smaller width when applicable.
 
-It differs from instcombine pass in that it contains pattern optimization that
-requires higher complexity than the O(1), thus, it should run fewer times than
-instcombine pass.
+It differs from instcombine pass in that it can modify CFG and contains pattern
+optimization that requires higher complexity than the O(1), thus, it should run fewer
+times than instcombine pass.
 
 ``-internalize``: Internalize Global Symbols
 --------------------------------------------
 
 This pass loops over all of the functions in the input module, looking for a
 main function.  If a main function is found, all other functions and all global
 variables with initializers are marked as internal.