Use switch expression for VarHandle$AccessMode lookup

JDK-8265079 changed jli.Invokers to pull in VarHandle$AccessMode on initialization, which then shows up as a small regression on a few startup tests due how AccessMode builds up a small HashMap on class initialization.

This map is read-only and could be converted to a switch expression. Doing so we marginally reduce the startup cost of initializing MHs/VHs and eliminate most of the regression from JDK-8265079