target/loongarch: Implement LASX fpu fcvt instructions

This patch includes:
- XVFCVT{L/H}.{S.H/D.S};
- XVFCVT.{H.S/S.D};
- XVFRINT[{RNE/RZ/RP/RM}].{S/D};
- XVFTINT[{RNE/RZ/RP/RM}].{W.S/L.D};
- XVFTINT[RZ].{WU.S/LU.D};
- XVFTINT[{RNE/RZ/RP/RM}].W.D;
- XVFTINT[{RNE/RZ/RP/RM}]{L/H}.L.S;
- XVFFINT.{S.W/D.L}[U];
- X[CVFFINT.S.L, VFFINT{L/H}.D.W.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230914022645.1151356-48-gaosong@loongson.cn>