[X86][CodeGen]Fix extract f16 from big vectors

[X86][CodeGen]Fix extract f16 from big vectors

When use llc -mattr=+avx512fp16, it will crash.

```
define half @test(<64 x half> %x, i64 %idx){
   %res = extractelement <64 x half> %x, i64 %idx
   ret half %res
}
```
The root cause is when we enable avx512fp16 we lose custom handler
for extract f16 from big vectors which is not loaded from pointer.

Reviewed By: pengfei

Differential Revision: https://reviews.llvm.org/D141348