Matcher::is_spillable_arg() seems to be obsolete and could be removed. It was still used in old releases like 8u :
https://github.com/search?q=repo%3Aopenjdk%2Fjdk8u+%22is_spillable_arg%22&type=code

Without removing it, and using standard build settings, the unused method still ends up in libjvm.so (at least on Linux) .

strings /jdk-opt/images/jdk/lib/server/libjvm.so | grep is_spillable_arg
_ZN7Matcher16is_spillable_argEi

When using linktime-gc, it is shown as eliminated from libjvm.so when printing of the eliminated sections is enabled :
  ....
/bin/ld: removing unused section '.text._ZN7Matcher16is_spillable_argEi' in file '/build_ltgc/hotspot/variant-server/libjvm/objs/ad_x86.o'

However linktime-gc still has some issues when used with the HS build, so it cannot easily enabled at the moment for libjvm on e.g. linux x86_64.