fold-const: Avoid infinite recursion in +-*&|^minmax reassociation [PR114084]

In the following testcase we infinitely recurse during BIT_IOR_EXPR
reassociation.
One operand is (unsigned _BitInt(31)) a << 4 and another operand
2147483647 >> 1 | 80 where both the right shift and the | 80
trees have TREE_CONSTANT set, but weren't folded because of delayed
folding, where some foldings are apparently done even in that case
unfortunately.
Now, the fold_binary_loc reassocation code splits both operands into
variable part, minus variable part, constant part, minus constant part,
literal part and minus literal parts, to prevent infinite recursion
punts if there are just 2 parts altogether from the 2 operands and then goes
on with reassociation, merges first the corresponding parts from both
operands and then some further merges.
The problem with the above expressions is that we get 3 different objects,
var0 (the left shift), con1 (the right shift) and lit1 (80), so the infinite
recursion prevention doesn't trigger, and we eventually merge con1 with
lit1, which effectively reconstructs the original op1 and then associate
that with var0 which is original op0, and associate_trees for that case
calls fold_binary.  There are some casts involved there too (the T typedef
type and the underlying _BitInt type which are stripped with STRIP_NOPS).

The following patch attempts to prevent this infinite recursion by tracking
the origin (if certain var comes from nothing - 0, op0 - 1, op1 - 2 or both - 3)
and propagates it through all the associate_tree calls which merge the vars.
If near the end we'd try to merge what comes solely from op0 with what comes
solely from op1 (or vice versa), the patch punts, because then it isn't any
kind of reassociation between the two operands, if anything it should be
handled when folding the suboperands.

2024-02-26  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/114084
	* fold-const.cc (fold_binary_loc): Avoid the final associate_trees
	if all subtrees of var0 come from one of the op0 or op1 operands
	and all subtrees of con0 come from the other one.  Don't clear
	variables which are never used afterwards.

	* gcc.dg/bitint-94.c: New test.