[X86] Avoid over-relaxation of 8-bit immediates in integer arithmetic instructions.

Only consider an instruction a candidate for relaxation if the last operand of the 
instruction is an expression. We previously checked whether any operand is an expression,
which is useless, since for all instructions concerned, the only operand that may be
affected by relaxation is the last one.
In addition, this removes the check for having RIP as an argument, since it was 
plain wrong - even when one of the arguments is RIP, relaxation may still be needed.

This fixes PR9807.

Patch by: david.l.kreitzer@intel.com
Differential Revision: http://reviews.llvm.org/D10766

llvm-svn: 241152