The X86FixupLEAs pass for Intel Atom must not call convertToThreeAddress
on ADD16rr opcodes, if src1 != src, since that would cause 
convertToThreeAddress to try to create a virtual register. This is not
permitted after register allocation, which is when the X86FixupLEAs pass
runs.

This patch fixes PR16785.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191711 91177308-0d34-0410-b5e6-96231b3b80d8