[clang] [llvm] [AMDGPU] Add support for `v_cos_bf16_e64` on gfx1250 (PR #149355)

[Shilei Tian via cfe-commits]

shiltian wrote:

but we do have `v_cos_bf16` in `llvm/test/MC/AMDGPU/gfx1250_asm_vop1.s`?

https://github.com/llvm/llvm-project/pull/149355