Optimizing: LoadString may not have any side effects.

LoadString does not have any side effects if the string is
known to be in the dex cache or it's a boot image string
referenced directly, as specified by the string load kind.
We need to clear the side effects for these cases to avoid
a DCHECK() failure when such LoadString instruction ends up
between a ClinitCheck and an instruction to which we want to
merge that ClinitCheck. This may happen as a consequence of
inlining, LICM and DCE as shown by a regression test.

Bug: 27929914

(cherry picked from commit ace7a000)

Change-Id: Iaf9c63b6e58aae1e246b43ca52eea0b47a6ad565