Auto merge of #122645 - saethlin:portable-mir-opt-tests, r=Nadrieril

Remove some only- clauses from mir-opt tests

Derived from rust-lang/rust#122295

Many of these tests were originally codegen tests, and MIR is more trivially portable than LLVM IR. We simply don't need to restrict the platform in most cases.

r? Nadrieril

Author: bors