[X86][BF16] Make backend type bf16 to follow the psABI

X86 psABI has updated to support __bf16 type, the ABI of which is the
same as FP16. See https://discourse.llvm.org/t/patch-add-optional-bfloat16-support/63149

This is an alternative of D129858, which has less code modification and
supports the vector type as well.

Reviewed By: LuoYuanke

Differential Revision: https://reviews.llvm.org/D130832