[InstCombine] Don't aggressively replace xor with icmp

For some cases, InstCombine replaces the sequence of xor/sub instruction
followed by cmp instruction into a single cmp instruction.

However, this replacement may result suboptimal result especially when
the xor/sub has more than one use, as discussed in
bug 26465 (https://llvm.org/bugs/show_bug.cgi?id=26465).

This patch make the replacement happen only when xor/sub has only one
use.

Differential Revision: http://reviews.llvm.org/D16915

Patch by Taewook Oh!

llvm-svn: 260695