ART: Refactor Int64ConstantFrom to use Int64FromConstant; rename it to Int64FromLocation

Int64ConstantFrom function duplicates code of the Int64FromConstant. Its
code can be replaced with a call: Int64FromConstant(location.getConstant()).

The patch removes the duplicating code. It also changes the function name to
Int64FromLocation to be consistent with its usage.

Test: test-art-host, test-art-target
Change-Id: I5624259aa72523f97ca8fc132a6152f338425c8e