rtl-ssa: Fix splitting of clobber groups [PR108508]

rtl-ssa: Fix splitting of clobber groups [PR108508]

Since rtl-ssa isn't a real/native SSA representation, it has
to honour the constraints of the underlying rtl representation.
Part of this involves maintaining an rpo list of definitions
for each rtl register, backed by a splay tree where necessary
for quick lookup/insertion.

However, clobbers of a register don't act as barriers to
other clobbers of a register.  E.g. it's possible to move one
flag-clobbering instruction across an arbitrary number of other
flag-clobbering instructions.  In order to allow passes to do
that without quadratic complexity, the splay tree groups all
consecutive clobbers into groups, with only the group being
entered into the splay tree.  These groups in turn have an
internal splay tree of clobbers where necessary.

This means that, if we insert a new definition and use into
the middle of a sea of clobbers, we need to split the clobber
group into two groups.  This was quite a difficult condition
to trigger during development, and the PR shows that the code
to handle it had (at least) two bugs.

First, the process involves searching the clobber tree for
the split point.  This search can give either the previous
clobber (which will belong to the first of the split groups)
or the next clobber (which will belong to the second of the
split groups).  The code for the former case handled the
split correctly but the code for the latter case didn't.

Second, I'd forgotten to add the second clobber group to the
main splay tree. :-(

gcc/
	PR rtl-optimization/108508
	* rtl-ssa/accesses.cc (function_info::split_clobber_group): When
	the splay tree search gives the first clobber in the second group,
	make sure that the root of the first clobber group is updated
	correctly.  Enter the new clobber group into the definition splay
	tree.

gcc/testsuite/
	PR rtl-optimization/108508
	* gcc.target/aarch64/pr108508.c: New test.