PR target/105991: Recognize PLUS and XOR forms of rldimi in rs6000.md.

PR target/105991: Recognize PLUS and XOR forms of rldimi in rs6000.md.

This patch addresses PR target/105991 where a change to prefer representing
shifts and adds at the tree-level as multiplications, causes problems for
the rldimi patterns in the powerpc backend.  The issue is that rs6000.md
models this pattern using IOR, and some variants that have the equivalent
PLUS or XOR in the RTL fail to match some *rotl<mode>4_insert patterns.
This is fixed in this patch by adding a define_insn_and_split to locally
canonicalize the PLUS and XOR forms to the backend's preferred IOR form.

An alternative fix might be for the RTL optimizers to define a canonical
form for these plus_xor_ior equivalent expressions, but the logical
choice might be plus (which may appear in an addressing mode), and such
a change may require a number of tweaks to update various backends
(i.e.  a more intrusive change than the one proposed here).

Many thanks for Marek Polacek for bootstrapping and regression testing
this change without problems.

2022-06-22  Roger Sayle  <roger@nextmovesoftware.com>
	    Marek Polacek  <polacek@redhat.com>
	    Segher Boessenkool  <segher@kernel.crashing.org>
	    Kewen Lin  <linkw@linux.ibm.com>

gcc/ChangeLog
	PR target/105991
	* config/rs6000/rs6000.md (rotl<mode>3_insert_3): Check that
	exact_log2 doesn't return -1 (or zero).
	(plus_xor): New code iterator.
	(*rotl<mode>3_insert_3_<code>): New define_insn_and_split.

gcc/testsuite/ChangeLog
	PR target/105991
	* gcc.target/powerpc/pr105991.c: New test case.