x86: Remove unnecessary temp from CheckCast locations.

This should have been removed in
    https://android-review.googlesource.com/300816 .
Also clean up related comments and refactor type checks
in the x86-64 codegen in line with the refactoring for
other architectures in that CL.

Test: testrunner.py --host --optimizing
Change-Id: I721338985e5388ecd9216e11bcd1c772de8416e6