[clang] [clang] fix half && bfloat16 convert node expr codegen (PR #89051)

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JinjinLi868 wrote:

> > But In some target, it supply a HW instruction to complete the process (fp16->float32->bf16) . so it just supply a intrinsic (fp16 -> bf16)
> 
> Which is not a bitcast. The correct IR representation of this conversion is fpext+fptrunc

i understand, i have changed. but for the vector convert case, it is also a bitcast IR

https://github.com/llvm/llvm-project/pull/89051