[x86] machine combiner reassociation: mark EFLAGS operand as 'dead'

In the commentary for D11660, I wasn't sure if it was alright to create new
integer machine instructions without also creating the implicit EFLAGS operand. 
From what I can see, the implicit operand is always created by the MachineInstrBuilder
based on the instruction type, so we don't have to do that explicitly. However, in
reviewing the debug output, I noticed that the operand was not marked as 'dead'. 
The machine combiner should do that to preserve future optimization opportunities 
that may be checking for that dead EFLAGS operand themselves.

Differential Revision: http://reviews.llvm.org/D11696

llvm-svn: 243990