Fix tests on LLVM 20 For sparcv8plus.rs, duplicate the test for LLVM 19 and LLVM 20. LLVM 20 resolves one of the FIXME in the test. For x86_64-bigint-add.rs split the check lines for LLVM 19 and LLVM 20. The difference in codegen here is due to a difference in unroll factor, which I believe is not what the test is interested in. Fixes https://github.com/rust-lang/rust/issues/132957. Fixes https://github.com/rust-lang/rust/issues/133754.