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 i16 %3, 32767
  %5 = icmp eq i16 %4, 0
  %6 = and i16 %3, -32768
  %broadcast.splatinsert = insertelement <4 x half> poison, half %2, i64 0
  %broadcast.splat = shufflevector <4 x half> %broadcast.splatinsert, <4 x half> poison, <4 x i32> zeroinitializer
  %broadcast.splatinsert9 = insertelement <4 x i16> poison, i16 %4, i64 0
  %broadcast.splat10 = shufflevector <4 x i16> %broadcast.splatinsert9, <4 x i16> poison, <4 x i32> zeroinitializer
  %broadcast.splatinsert11 = insertelement <4 x i16> poison, i16 %6, i64 0
  %broadcast.splat12 = shufflevector <4 x i16> %broadcast.splatinsert11, <4 x i16> poison, <4 x i32> zeroinitializer
  %broadcast.splatinsert13 = insertelement <4 x i16> poison, i16 %3, i64 0
  %broadcast.splat14 = shufflevector <4 x i16> %broadcast.splatinsert13, <4 x i16> poison, <4 x i32> zeroinitializer
  %wide.load = load <4 x half>, ptr %Arg_1.2, align 8
  %7 = fcmp uno <4 x half> %broadcast.splat, %wide.load
  %8 = fcmp oeq <4 x half> %broadcast.splat, %wide.load
  %9 = bitcast <4 x half> %wide.load to <4 x i16>
  %10 = and <4 x i16> %9, <i16 32767, i16 32767, i16 32767, i16 32767>
  %11 = icmp eq <4 x i16> %10, zeroinitializer
  %12 = and <4 x i16> %9, <i16 -32768, i16 -32768, i16 -32768, i16 -32768>
  %13 = or <4 x i16> %12, <i16 1, i16 1, i16 1, i16 1>
  %14 = select <4 x i1> %11, <4 x i16> %9, <4 x i16> %13
  %15 = icmp ugt <4 x i16> %broadcast.splat10, %10
  %16 = icmp ne <4 x i16> %broadcast.splat12, %12
  %17 = or <4 x i1> %15, %16
  %18 = select <4 x i1> %17, <4 x i16> <i16 -1, i16 -1, i16 -1, i16 -1>, <4 x i16> <i16 1, i16 1, i16 1, i16 1>
  %19 = add <4 x i16> %18, %broadcast.splat14
  %20 = select i1 %5, <4 x i16> %14, <4 x i16> %19
  %21 = select <4 x i1> %8, <4 x i16> %9, <4 x i16> %20
  %22 = bitcast <4 x i16> %21 to <4 x half>
  %23 = select <4 x i1> %7, <4 x half> <half 0xH7E00, half 0xH7E00, half 0xH7E00, half 0xH7E00>, <4 x half> %22
  store <4 x half> %23, ptr %fusion, align 16
  ret void
}
```

llc: llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp:977: void (anonymous namespace)::SelectionDAGLegalize::LegalizeOp(llvm::SDNode *): Assertion `(TLI.getTypeAction(*DAG.getContext(), Op.getValueType()) == TargetLowering::TypeLegal || Op.getOpcode() == ISD::TargetConstant || Op.getOpcode() == ISD::Register) && "Unexpected illegal type!"' failed.