[llvm] [AVR] Fix a bug in selection of ANY_EXTEND (PR #132398)

[Patryk Wychowaniec via llvm-commits]

https://github.com/Patryk27 requested changes to this pull request.

For others - the issue is that we end up generating a 16-bit instruction (subtraction, addition, ...) on a partially initialized register-pair.

Following my simplified example, after avr-isel we get:
```
bb.0 (%ir-block.0):
  liveins: $r24
  %0:gpr8 = COPY $r24
  %2:dregs = IMPLICIT_DEF
  %1:dldregs = INSERT_SUBREG %2:dregs(tied-def 0), %0:gpr8, %subreg.sub_lo
  %3:dldregs = SUBIWRdK %1:dldregs(tied-def 0), 256, implicit-def dead $sreg
  %4:dldregs = ANDIWRdK %3:dldregs(tied-def 0), 1, implicit-def dead $sreg
  $r25r24 = COPY %4:dldregs
  RET implicit $r25r24, implicit $r1
```
... which later gets expanded into:
```
0B	bb.0 (%ir-block.0):
	  liveins: $r24
16B	  undef %4.sub_lo:dldregs = COPY $r24
64B	  %4:dldregs = SUBIWRdK %4:dldregs(tied-def 0), 256, implicit-def dead $sreg
96B	  %4:dldregs = ANDIWRdK %4:dldregs(tied-def 0), 1, implicit-def dead $sreg
112B	  $r25r24 = COPY %4:dldregs
128B	  RET implicit $r25r24, implicit $r1
```
... and into (via `virtregrewriter` - note the lost information about partially initialized `$r25r24`):
```
0B	bb.0 (%ir-block.0):
	  liveins: $r24
64B	  $r25r24 = SUBIWRdK $r25r24(tied-def 0), 256, implicit-def dead $sreg
96B	  $r25r24 = ANDIWRdK killed $r25r24(tied-def 0), 1, implicit-def dead $sreg
128B	  RET implicit $r25r24, implicit $r1
```
... into:
```
bb.0 (%ir-block.0):
  liveins: $r24
  $r24 = SUBIRdK $r24(tied-def 0), 0, implicit-def $sreg
  $r25 = SBCIRdK $r25(tied-def 0), 1, implicit-def dead $sreg, implicit killed $sreg
  $r24 = ANDIRdK killed $r24(tied-def 0), 1, implicit-def dead $sreg
  $r25 = ANDIRdK killed undef $r25(tied-def 0), 0, implicit-def dead $sreg
  RET implicit $r25r24, implicit $r1
```

Note that the second `ANDIRdK` correctly says `undef $25`, but that's just a happy accident thanks to
https://github.com/llvm/llvm-project/blob/f3e8e805630e5092458a80e00ba83f7ad45e2302/llvm/lib/Target/AVR/AVRExpandPseudoInsts.cpp#L281

tl;dr it looks like somewhere between avr-isel and virtregrewriter we lose the information about "undef-ness"; I'm not yet sure what's the correct solution, but the proposed hotfix seems to be correct.

https://github.com/llvm/llvm-project/pull/132398