tests/assembly/x86_64-bigint-add.rs broken with LLVM 20

[krasimirgg]

It looks like tests/assembly/x86_64-bigint-add.rs broken, newly added in 9836196, produces slightly different assembly with LLVM-at-HEAD:

https://buildkite.com/llvm-project/rust-llvm-integrate-prototype/builds/33017#01938775-555b-4915-9c10-08f40d14cdad/945-946

@scottmcm: does the new assembly produced look good? Happy to adapt the test for this.

cc: @scottmcm @durin42

[krasimirgg]

@rustbot label +A-LLVM

[nikic]

It looks like this is just a difference in unroll factor: https://www.diffchecker.com/qUdwbutN/ LLVM 20 unrolls this loop less aggressively.