Request for review (S): 6833879 Assigning positive zero is ignored when old value is negative zero

[Changpeng Fang]

http://cr.openjdk.java.net/~cfang/6833879/webrev.00/

Problem:
 For the following case:
 public static double normalize(float v) {
        if (v == 0.0f) v = 0.0f;
        return v;
}
If -0.0 is passed, 0.0 is expected to be returned (it is the case for 
the interpreter). However,
PhiNode::Identity optimized away the "if and assignment" statement and 
the server vm
return -0.0.

Proposed Solution:
Don't perform this kind of identity optimization if either true path or 
false path is constant
0.0 or (-0.0)

Open Issue:
When the true path or false path is not constant, it may still be 0.0 
(-0.0) at run time.
Do we have to turn off this identity optimization completely  for float  
and double (except
for constant non-zeros)? 

Tests: Passed the test case in the CR report.

Thanks,

Changpeng