ART: Simplify invoke-polymorphic entrypoints

Moves to pattern used by the interpreter bridge and writes the result
in both the regular return register and the floating point result
register.

Add return value tests to 956-method-handles.

Test: art/test.py --host -r -t 956
Test: art/test.py --target --32 -r -t 956
Test: art/test.py --target --64 -r -t 956
Change-Id: I7389d04b70b88e149682f6d656ab185e48bcbf66