Optimizing: LoadString may not have any side effects.

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
Change-Id: I7b3bddf7d8c79ce1828a4a751f1270cf2e3d61f0