[LiveIntervals] Improve repair after convertToThreeAddress

After TwoAddressInstructionPass calls
TargetInstrInfo::convertToThreeAddress, improve the LiveIntervals repair
to cope with convertToThreeAddress creating more than one new
instruction.

This mostly seems to benefit X86. For example in
test/CodeGen/X86/zext-trunc.ll it converts:

  %4:gr32 = ADD32rr %3:gr32(tied-def 0), %2:gr32, implicit-def dead $eflags

to:

  undef %6.sub_32bit:gr64 = COPY %3:gr32
  undef %7.sub_32bit:gr64_nosp = COPY %2:gr32
  %4:gr32 = LEA64_32r killed %6:gr64, 1, killed %7:gr64_nosp, 0, $noreg

Differential Revision: https://reviews.llvm.org/D110335