RFR(XS): 8209684: Intrinsics that assume some input non null should use GraphKit::must_be_not_null()

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8209684/webrev.00/

A number of intrinsics omit null checks on inputs that are known to be
not null. They should mark these inputs as not null instead with
GraphKit::must_be_not_null().

This change is from the shenandoah repo where it's required:
shenandoah's barrier read a field from the object. If the object is not
registered to be not null then this field load can turn on some path
into a load from a null object which causes C2 to crash.

Roland.