[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:s...