Faster class retrieval in nterp.

Split `NterpGetClassOrAllocateObject` to two individual
functions to avoid a slow `switch` and an `if`.

Also deduplicate exception throwing code in nterp.

Test: testrunner.py --host --interpreter --jit
Test: testrunner.py --target --interpreter --jit
Change-Id: I927af8f534c43d0bd760f555c0de1dd220f7df93
16 files changed