x86: optimize LEA

x86: optimize LEA

Over the years I've seen a number of instances where people used

    lea     (%reg1), %reg2

or

    lea     symbol, %reg

despite the same thing being expressable via MOV. Since additionally
LEA often has restrictions towards the ports it can be issued to, while
MOV typically gets dealt with simply by register renaming, transform to
MOV when possible (without growing opcode size and without altering
involved relocation types).

Note that for Mach-O the new 64-bit testcases would fail (for
BFD_RELOC_X86_64_32S not having a representation), and hence get skipped
there.