ir3/ra: Fix bug with collect source handling

It can be the case that a collect and one of its sources are assigned
to non-overlapping parts of the same merge set, for example:

	ssa_1 = ...
	ssa_2 = ...
	ssa_3 = ...

	ssa_4 = collect ssa_1, ssa_2 (kill), ssa_3
	... = ssa_4 (kill)

	ssa_5 = collect ssa_1, ssa_3

	... = ssa_1 (kill)
	... = ssa_3 (kill)
	... = ssa_5 (kill)

If we merge the first collect first, we get a merge set:

	ssa_1 (offset 0)
	ssa_2 (offset 2)
	ssa_3 (offset 4)
	ssa_4 (offset 0)

Now, we decide to merge ssa_1 and ssa_5:

	ssa_1 (offset 0)
	ssa_2 (offset 2)
	ssa_3 (offset 4)
	ssa_4 (offset 0)
	ssa_5 (offset 0)

ssa_3 cannot become a child of ssa_5 in the interval tree, just like a
source not in the same merge set, so we should not remove it and then
reinsert it assuming that RA will make it a child of ssa_5.

This fixes an RA validation error in Farming Simulater.

Fixes: 0ffcb19b ("ir3: Rewrite register allocation")
Part-of: <mesa/mesa!27497>
(cherry picked from commit aeed5fd9)