RFR 8226860 [lworld][c1] runtime/valhalla/valuetypes/Ifacmp.java fails with C1

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8226860
http://cr.openjdk.java.net/~iklam/valhalla/8226860-runtime-ifacmp-test-fails-with-c1.v01/

In C1, substitutability check was implemented only in 
LIRGenerator::do_IfOp().
This patch implements it also in LIRGenerator::do_If().

I also fixed a bug in SubstitutabilityCheckStub which would 
unintentionally scratch
the RAX register.

Thanks
- Ioi