[LoongArch] Support CodeModel::Large codegen

This is intended to behave like GCC's `-mcmodel=extreme`.

Technically the true GCC equivalent would be `-mcmodel=large` which is
not yet implemented there, and we probably do not want to take the
"Large" name until things settle in GCC side, but:

* LLVM does not have a `CodeModel::Extreme`, and it seems too early to
  have such a variant added just for enabling LoongArch; and
* `CodeModel::Small` is already being used for GCC `-mcmodel=normal`
  which is already a case of divergent naming.

Regarding the codegen, loads/stores immediately after a PC-relative
large address load (that ends with something like `add.d $addr, $addr,
$tmp`) should get merged with the addition into corresponding `ldx/stx`
ops, but is currently not done. This is because pseudo-instructions are
expanded after instruction selection, and is best fixed with a separate
change.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D150522