[x86] remove dead code for 16-bit LEA formation; NFC

As discussed in:
D55494
...this code has been disabled/dead for a long time (the code references
Athlon and Pentium 4), and there's almost no chance that it will be used 
given the last decade of uarch evolution. Also, in SDAG we promote 16-bit 
ops to 32-bit, so there's almost no way to test this code any more.

llvm-svn: 348845