[llvm] [X86][GISel] Fix crash on bitcasting i16 <-> half with gisel enabled. (PR #168456)

[Evgenii Kudriashov via llvm-commits]

e-kud wrote:

I tend to consider that GlobalISel needs to emit quite similar code to SDAG on such small examples.
```
Optimized legalized selection DAG: %bb.0 'test_i16_to_half:entry'
SelectionDAG has 9 nodes:
  t0: ch,glue = EntryToken
        t2: i32,ch = CopyFromReg t0, Register:i32 %0
      t3: i16 = truncate t2
    t4: f16 = bitcast t3
  t7: ch,glue = CopyToReg t0, Register:f16 $xmm0, t4
  t8: ch = X86ISD::RET_GLUE t7, TargetConstant:i32<0>, Register:f16 $xmm0, t7:1
```
```
# *** IR Dump Before InstructionSelect (instruction-select) ***:
# Machine code for function test_i16_to_half: IsSSA, TracksLiveness, Legalized, RegBankSelected
Function Live Ins: $edi

bb.1.entry:
  liveins: $edi
  %1:gpr(s32) = COPY $edi
  %0:gpr(s16) = G_TRUNC %1:gpr(s32)
  $xmm0 = COPY %0:gpr(s16)
  RET 0, implicit $xmm0
```

I'd think that the problem is in GlobalISel that bitcast hasn't been inserted by IRTranslator. But probably for natural insertion of this bitcast #155107 is required.

https://github.com/llvm/llvm-project/pull/168456