Fix a problem with incomplete equality testing of PHINodes in

Fix a problem with incomplete equality testing of PHINodes in
Instruction::IsIdenticalToWhenDefined.

This manifested itself when inlining two calls to the same function.  The
inlined function had a switch statement that returned one of a set of
global variables.  Without this modification, the two phi instructions that
chose values from the branches of the switch instruction inlined from the
callee were considered equivalent and jump-threading replaced a load for the
first switch value with a phi selecting from the second switch, thereby
producing incorrect code.

This patch has been tested with "make check-all", "lnt runteste nt", and
llvm self-hosted, and on the original program that had this problem,
wireshark.

<rdar://problem/11025519>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156548 91177308-0d34-0410-b5e6-96231b3b80d8