Request for review(XXS): 6946892 c1 shouldn't sign-extend to upper 32bits on x64

[Igor Veresov]

C1 does sign-extension when it loads ints and shorts from memory to 
64-bit registers. This causes problems for C2 because it relies on the 
fact the int passed in a 64-bit register is zero-extended.

Webrev: http://cr.openjdk.java.net/~iveresov/6946892/webrev.00/

Thanks,
igor