[x86] don't try to convert add with undef operands to LEA

The existing code tries to handle an undef operand while transforming an add to an LEA, 
but it's incomplete because we will crash on the i16 test with the debug output shown below. 
It's better to just give up instead. Really, GlobalIsel should have folded these before we 
could get into trouble.

# Machine code for function add_undef_i16: NoPHIs, TracksLiveness, Legalized, RegBankSelected, Selected

bb.0 (%ir-block.0):
  liveins: $edi
  %1:gr32 = COPY killed $edi
  %0:gr16 = COPY %1.sub_16bit:gr32
  %5:gr64_nosp = IMPLICIT_DEF
  %5.sub_16bit:gr64_nosp = COPY %0:gr16
  %6:gr64_nosp = IMPLICIT_DEF
  %6.sub_16bit:gr64_nosp = COPY %2:gr16
  %4:gr32 = LEA64_32r killed %5:gr64_nosp, 1, killed %6:gr64_nosp, 0, $noreg
  %3:gr16 = COPY killed %4.sub_16bit:gr32
  $ax = COPY killed %3:gr16
  RET 0, implicit killed $ax

# End machine code for function add_undef_i16.

*** Bad machine code: Reading virtual register without a def ***
- function:    add_undef_i16
- basic block: %bb.0  (0x7fe6cd83d940)
- instruction: %6.sub_16bit:gr64_nosp = COPY %2:gr16
- operand 1:   %2:gr16
LLVM ERROR: Found 1 machine code errors.

Differential Revision: https://reviews.llvm.org/D54710

llvm-svn: 348722