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

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