[GlobalISel] Remove non-determinism from IRTranslator.

This showed up in r300535/r300537, which were reverted in r300538 due to
some of the introduced tests in there failing on some bots, due to the
non-determinism fixed in this commit.

Re-committing r300535/r300537 will add 2 tests for the change in this
commit.

llvm-svn: 300663