Auto merge of #111768 - oli-obk:pair_const_llvm, r=cjgillot

Optimize scalar and scalar pair representations loaded from ByRef in llvm

in rust-lang/rust#105653 I noticed that we were generating suboptimal LLVM IR if we had a `ConstValue::ByRef` that could be represented by a `ScalarPair`. Before rust-lang/rust#105653 this is probably rare, but after it, every slice will go down this suboptimal code path that requires LLVM to untangle a bunch of indirections and translate static allocations that are only used once to read a scalar pair from.

Author: bors