testsuite: riscv: adjust/xfail dynamic-lmul tests

While investigating riscv*-elf testsuite failures in gcc-15, I found
mainline still had failures for a number of the dynamic-lmul tests I
investigated, though not always the same as gcc-15 due to other
changes.  Here are some adjustments to the tests, whether updates or
xfails, to fix or silence the failures for gcc-16, respectively.

* dynamic-lmul4-3.c: r16-843 had bumped the "Maximum lmul = 2" count,
which was reasonable then, but subsequent changes in e.g. r16-2515
restored the original count, so revert that expectation.

* dynamic-lmul4-7.c: SLP improvements seem to have caused us to find
more live V_REGs during analysis, so we choose 1 instead of 2 for
lmul.  Because of additional live V_REGs, AFAICT introduced by SLP, we
lower the RVV mode choice twice rather than the expected once.  Since
the SLP improvements AFAICT make the code worse, at least for leading
to a lower lmul choice, I'm marking these two deviations as expected
failures.

* dynamic-lmul4-5.c: Like 4-7, this one regressed in the lmul choice
due to extra live V_REGs, and thus got extra messages about preferring
a lower lmul.  As for the start/end patterns, I can't tell what they
were meant to test, but the live V_REGs have changed so significantly,
and are so likely to keep changing, that I figured we're better off
dropping those failing expectations.  This is the only test that had
them.

* dynamic-lmul4-6.c: For some reason I haven't tried to figure out,
but I guess it also has to do with SLP improvements, the "Maximum lmul
= [24]" messages are no longer printed after "Update local program
points for bb 6" for this test, so I'm XFAILing them, because that
feels like a regression even though we end up with the expected lmul.

* dynamic-lmul4-8.c: Same as 4-6.

* dynamic-lmul8-10.c: I haven't tried to figure out why the initial
maximum lmul estimate has fallen from 8 to 4, despite the V_REG's
count being (proportionally) the same and fitting, but we still choose
8 for lmul, so I'm dropping the requirement for an 'lmul = 8' match
under the tentative conclusion that the expectation is unnecessary.

* dynamic-lmul8-12.c: Same as 8-10 and 4-6.

* pr113112-3.c: We get a much higher lmul (m8) than expected (m2), for
the same e32 element size, but the generated code is far too much
larger and hairier.  While the lmul bump is probably a welcome
improvement, the expected choice of return instruction isn't found,
and I can't tell why the test prefers one over the other, so I'm
XFAILing all of them.

* pr113112-4.c: We get a lower lmul than expected, but that
expectation was different before r15-639, and we get what we used to
expect before that change.  Since we could do better at some point,
I'm XFAILing it.


for  gcc/testsuite/ChangeLog

	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-3.c: Revert
	Maximum lmul = 2 count.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-7.c: XFAIL
	unmet lmul selection expectations.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-5.c: Likewise,
	and drop start/end patterns.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-6.c: XFAIL
	missing Maximum lmul messages.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul4-8.c: Likewise.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-12.c: Drop
	Maximum lmul = 8 message.  Likewise.
	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul8-10.c: Likewise.
	* gcc.dg/vect/costmodel/riscv/rvv/pr113112-3.c: XFAIL unexpected
	selections of lmul, and of return insn.
	* gcc.dg/vect/costmodel/riscv/rvv/pr113112-4.c: XFAIL unexpected
	selection of lmul.

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