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>

llvm-svn: 156548