[X86] Fix killed flag handling in X86FixupLea pass

When pass creates a MOV instruction for 
lea (%base,%index,1), %dst => mov %base,%dst; add %index,%dst
modification it should clean the killed flag for base
if base is equal to index.

Otherwise verifier complains about usage of killed register in add instruction.

Reviewers: lsaba, zvi, zansari, aaboud
Reviewed By: lsaba
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D42522

llvm-svn: 323497