RISC-V: Leverage vaadd.vv for signed standard name avg_floor

The signed avg_floor totally match the sematics of fixed point
rvv insn vaadd, within round down.  Thus, leverage it directly
to implement the avf_floor.

The spec of RVV is somehow not that clear about the difference
between the float point and fixed point for the rounding that
discard least-significant information.

For float point which is not two's complement, the "discard
least-significant information" indicates truncation round.  For
example as below:

* 3.5 -> 3
* -2.3 -> -2

For fixed point which is two's complement, the "discard
least-significant information" indicates round down.  For
example as below:

* 3.5 -> 3
* -2.3 -> -3

And the vaadd takes the round down which is totally matching
the sematics of the avf_floor.

The below test suites are passed for this patch series.
* The rv64gcv fully regression test.

gcc/ChangeLog:

	* config/riscv/autovec.md (avg<v_double_trunc>3_floor): Add insn
	expand to leverage vaadd directly.

Signed-off-by: Pan Li <pan2.li@intel.com>

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