RFR: 8173472: AArch64: C1 comparisons with null only use 32-bit instructions

[Andrew Haley]

Yesterday I found an absolute doozy of a bug: we're only comparing 32
bits of an address with null.  So a couple of times in a billion, if
the heap is in the right place and the moon is in the right phase, a
null pointer comparison will result in a false positive.

http://cr.openjdk.java.net/~aph/8173472-1/

Andrew.