Revert r42653 and forward-port the code that lets INC64_32r be

converted to LEA64_32r in x86's convertToThreeAddress. This
replaces code like this:
   movl  %esi, %edi
   inc   %edi
with this:
   lea   1(%rsi), %edi
which appears to be beneficial.

llvm-svn: 61830