RFR: missing must_be_not_null() for arguments to String compareTo*/equals*

[Roland Westrelin]

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

This fixes:

#  Internal Error (/home/jenkins/workspace/nightly/shenandoah-jdk10/src/hotspot/share/opto/compile.cpp:1740), pid=85566, tid=85590
#  assert(flat != TypePtr::BOTTOM) failed: cannot alias-analyze an untyped ptr: adr_type = NULL+12

Arguments to intrinsics are known to be non null.

Roland.