[ELF] Fix large code model MIR verifier errors

Instead of using the MOVGOT64r pseudo, use the existing
MO_PIC_BASE_OFFSET support on symbol operands. Now I don't have to
create a "scratch register operand" for the pseudo to use, and the
register allocator can make better decisions.

Fixes some X86 verifier errors tracked in PR27481.

llvm-svn: 345219