Ignore allocation bytes in some mir-opt tests This adds `rustc -Zdump-mir-exclude-alloc-bytes` to skip writing allocation bytes in MIR dumps, and applies it to tests that were failing on s390x due to its big-endian byte order. Fixes #126261
The goal of this directory is to track the quality of MIR that is given to codegen in a standard -O
condiguration.
As such, feel free to --bless
whatever changes you get here, so long as doing so doesn't add substantially more MIR.