Currently, TargetLowering::sortVarsByAlignment() uses the variable's required alignment as the primary key, but then std::sort() breaks ties arbitrarily.  This can give different results across different STL implementations, for example when building against LLVM trunk versus building in the PNaCl environment.

The fix is to use Variable::Number as the secondary key.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/2295393002 .