Fix merging HLoadClass with HNewInstance.

Do not merge HLoadClass with HNewInstance if they do not
come from the same dex instruction. This is the same check
as for moving the clinit check responsibility around.

Test: Added a regression test, run standard ART test suite
plus gcstress and ndebug modes on host and Nexus 9.

Bug: 29570861
Bug: 29321958

(cherry picked from commit c7591b4c)

Change-Id: I0f019c36dd991889caad2666e61bbcc6d9d9c91f