[AMDGPU] Handle inUndef flag in LiveVariables::recomputeForSingleDefVirtReg

A register's use with isUndef flags shouldn't be considered as a point where the register is live. LiveVariables::runOnInstr ignores such uses.

This was found when I tried to replace calls to

  SIOptimizeVGPRLiveRange::updateLiveRangeInThenRegion
  SIOptimizeVGPRLiveRange::updateLiveRangeInElseRegion

with LiveVariables::recomputeForSingleDefVirtReg.

In the testcase below %2 use is undef in the last REG_SEQUENCE.

CodeGen/AMDGPU/si-opt-vgpr-liverange-bug-deadlanes.mir failed:

Function Live Ins: $vgpr0 in %0

bb.0:
  successors: %bb.1(0x40000000), %bb.2(0x40000000); %bb.1(50.00%), %bb.2(50.00%)
  liveins: $vgpr0
  %0:vgpr_32 = COPY killed $vgpr0
  %1:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
  %2:vgpr_32 = BUFFER_LOAD_DWORD_OFFEN killed %0:vgpr_32, undef %5:sgpr_128, 0, 0, 0, 0, implicit $exec :: (dereferenceable invariant load (s32))
  %3:sreg_64 = V_CMP_NE_U32_e64 0, %2:vgpr_32, implicit $exec
  %7:sreg_64 = SI_IF killed %3:sreg_64, %bb.2, implicit-def dead $exec, implicit-def dead $scc, implicit $exec
  S_BRANCH %bb.1

bb.1:
; predecessors: %bb.0
  successors: %bb.2(0x80000000); %bb.2(100.00%)

  %8:vreg_128 = REG_SEQUENCE killed %1:vgpr_32, %subreg.sub0, %1:vgpr_32, %subreg.sub1, %1:vgpr_32, %subreg.sub2, undef %4.sub3:vreg_128, %subreg.sub3

bb.2:
; predecessors: %bb.0, %bb.1
  successors: %bb.3(0x40000000), %bb.4(0x40000000); %bb.3(50.00%), %bb.4(50.00%)

  %9:vreg_128 = PHI undef %10:vreg_128, %bb.0, %8:vreg_128, %bb.1
  %14:vgpr_32 = PHI %2:vgpr_32, %bb.0, undef %15:vgpr_32, %bb.1
  %11:sreg_64 = SI_ELSE killed %7:sreg_64, %bb.4, implicit-def dead $exec, implicit-def dead $scc, implicit $exec
  S_BRANCH %bb.3

bb.3:
; predecessors: %bb.2
  successors: %bb.4(0x80000000); %bb.4(100.00%)

  %12:vreg_128 = REG_SEQUENCE killed %14:vgpr_32, %subreg.sub0, %14:vgpr_32, %subreg.sub1, %14:vgpr_32, %subreg.sub2, undef %6:vgpr_32, %subreg.sub3

bb.4:
; predecessors: %bb.2, %bb.3

  %13:vreg_128 = PHI %9:vreg_128, %bb.2, %12:vreg_128, %bb.3
  SI_END_CF killed %11:sreg_64, implicit-def dead $exec, implicit-def dead $scc, implicit $exec
  dead %4:vreg_128 = REG_SEQUENCE killed %13.sub2:vreg_128, %subreg.sub0, %13.sub2:vreg_128, %subreg.sub1, %13.sub2:vreg_128, %subreg.sub2, **undef %2:vgpr_32**, %subreg.sub3
  S_ENDPGM 0

*** Bad machine code: LiveVariables: Block should not be in AliveBlocks ***
- function:    _amdgpu_ps_main
- basic block: %bb.1  (0x55e17ebd7100)
Virtual register %2 is not needed live through the block.

*** Bad machine code: LiveVariables: Block should not be in AliveBlocks ***
- function:    _amdgpu_ps_main
- basic block: %bb.2  (0x55e17ebd7200)
Virtual register %2 is not needed live through the block.

*** Bad machine code: LiveVariables: Block should not be in AliveBlocks ***
- function:    _amdgpu_ps_main
- basic block: %bb.3  (0x55e17ebd7300)
Virtual register %2 is not needed live through the block.

Differential Revision: https://reviews.llvm.org/D158167