Let llvm-diff correctly deal with Undef/ConstantAggregateZero/ConstantVector/IndirectBr

Summary:
llvm-diff incorrectly reports that there's a diff when input IR contains undef/zeroinitializer/constantvector/indirectbr.
(This happens even if two identical files are given, e.g. `llvm-diff x.ll x.ll`)

This is fix to the bug report https://bugs.llvm.org/show_bug.cgi?id=33623 .



Reviewers: dexonsmith, rjmccall

Reviewed By: rjmccall

Subscribers: chenwj, mgrang, llvm-commits

Differential Revision: https://reviews.llvm.org/D34856

llvm-svn: 329957