[X86] Optimization for replacing LEA with MOV at frame index elimination time

Summary:
Replace a LEA instruction of the form 'lea (%esp), %ebx' --> 'mov %esp, %ebx'

MOV is preferable over LEA because usually there are more issue-slots available to execute MOVs than LEAs. Latest processors also support zero-latency MOVs.

Fixes pr29022.

Reviewers: hfinkel, delena, igorb, myatsina, mkuper

Differential Revision: https://reviews.llvm.org/D24705

llvm-svn: 282385