[PATCH v1 1/1] RISC-V: Nan-box the result of movbf on soft-bf16

[PATCH v1 1/1] RISC-V: Nan-box the result of movbf on soft-bf16

1 This patch implements the Nan-box of bf16.

2 Please refer to the Nan-box implementation of hf16 in:
<https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=057dc349021660c40699fb5c98fd9cac8e168653>

3 The discussion about Nan-box can be found on the website:
<https://www.mail-archive.com/search?q=Nan-box+the+result+of+movhf+on+soft-fp16&l=gcc-patches%40gcc.gnu.org>

4 Below test are passed for this patch
        * The riscv fully regression test.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_legitimize_move): Expand movbf
	with Nan-boxing value.
	* config/riscv/riscv.md (*movbf_softfloat_boxing): New pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/_Bfloat16-nanboxing.c: New test.

(cherry picked from commit ce51e6727c9d69bbab0e766c449e60fd41f5f2f9)