Revert "Reland "Reland "Reland "[X86][RFC] Enable `_Float16` type support on...

Revert "Reland "Reland "Reland "[X86][RFC] Enable `_Float16` type support on X86 following the psABI""""

This reverts commit 04a3d5f3.

I see two more issues:

- uitofp/sitofp from i32/i64 to half now generates
  __floatsihf/__floatdihf, which exists in neither compiler-rt nor
  libgcc

- This crashes when legalizing the bitcast:
```
; RUN: llc < %s -mcpu=skx
define void @main.45(ptr nocapture readnone %retval, ptr noalias nocapture readnone %run_options, ptr noalias nocapture readnone %params, ptr noalias nocapture readonly %buffer_table, ptr noalias nocapture readnone %status, ptr noalias nocapture readnone %prof_counters) local_unnamed_addr {
entry:
  %fusion = load ptr, ptr %buffer_table, align 8
  %0 = getelementptr inbounds ptr, ptr %buffer_table, i64 1
  %Arg_1.2 = load ptr, ptr %0, align 8
  %1 = getelementptr inbounds ptr, ptr %buffer_table, i64 2
  %Arg_0.1 = load ptr, ptr %1, align 8
  %2 = load half, ptr %Arg_0.1, align 8
  %3 = bitcast half %2 to i16
  %4 = and...