[llvm] [LoongArch][GlobalISel] Adding initial GlobalISel infrastructure (PR #76912)

[Amara Emerson via llvm-commits]

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+; RUN: llc -O0 --mtriple=loongarch32 -global-isel -stop-after=irtranslator -verify-machineinstrs < %s \
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aemerson wrote:

The standard option for GlobalISel is `-global-isel` with a single dash.

https://github.com/llvm/llvm-project/pull/76912