Add isInitializing check in nterp header.

This simplifies Instrumentation::GetCodeForInvoke and will be consistent
with the code we generate for AOT in:
https://android-review.googlesource.com/c/platform/art/+/2163021

Bug: 162110941
Test: test.py
Change-Id: Ic4d7747c7729df1281a2ab309589005c41542a31
10 files changed