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

[Matt Arsenault via cfe-commits]

arsenm 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 

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