[x86] allow 8-bit adds to be promoted by convertToThreeAddress() to form LEA

This extends the code that handles 16-bit add promotion to form LEA to also allow 8-bit adds. 
That allows us to combine add ops with register moves and save some instructions. This is 
another step towards allowing add truncation in generic DAGCombiner (see D54640).

Differential Revision: https://reviews.llvm.org/D55494

llvm-svn: 348946