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

[via llvm-commits]

heiher wrote:

## rustc-perf

### Compile-time instructions

|     |        Range       |  Mean  |  Count  |
|:---:|:------------------:|:------:|:-------:|
|   ❌ | [ +0.72%,  +0.72%] | +0.72% |   1 (1) |
|   ✅ | [ -1.40%,  -0.30%] | -0.58% | 71 (23) |
| ❌,✅ | [ -1.40%,  +0.72%] | -0.56% | 72 (23) |

### Artifact size

|      Component      |   Kind  |     Before |      After |      Change | % Change |
|:-------------------:|:-------:|-----------:|-----------:|------------:|---------:|
|        cargo        |  Binary |  35.58 MiB |  35.21 MiB | -381.62 KiB |  -1.047% |
|        rustc        |  Binary |   1.48 MiB |   1.48 MiB |         8 B |   0.001% |
|       rustdoc       |  Binary |  13.33 MiB |  13.24 MiB |  -95.69 KiB |  -0.701% |
|     libLLVM.so      | Library | 166.96 MiB | 166.97 MiB |    3.17 KiB |   0.002% |
| librustc_driver.so  | Library | 104.37 MiB | 103.95 MiB | -431.25 KiB |  -0.404% |
|      libstd.so      | Library |   2.73 MiB |   2.72 MiB |  -15.06 KiB |  -0.539% |

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