[llvm] [LoongArch] Introduce PseudoAddUpperImm to deduplicate stack-frame base address materialization (PR #217250)

[via llvm-commits]

lrzlin wrote:

The improvement will shrink to 6% if we compiling `709.cactus_r` with `-O3 -mlsx` instead of `-O3 -mlasx`, however the score increased from 2.18 to 2.31, which is higher than optimized LASX one.

https://github.com/llvm/llvm-project/pull/217250