The lower 32 bits of output FPURegister is undefined now, this CL
copies the input FPURegister's lower 32 bits to output FPURegister.

Change-Id: I10c078fafeddd5de207ced4f7c01f35d32999733
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2449153Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Cr-Commit-Position: refs/heads/master@{#70302}