Rollup merge of #106046 - uweigand:s390x-test-bigendian-mir, r=Mark-Simulacrum

Rollup merge of #106046 - uweigand:s390x-test-bigendian-mir, r=Mark-Simulacrum

Fix mir-opt tests for big-endian platforms

The test cases src/test/mir-opt/building/custom/consts.rs and src/test/mir-opt/const_prop/mutable_variable_no_prop.rs are currently failing on big-endian platforms as the binary encoding of some constants is hard-coded in the MIR test files.  Fix this by choosing constant values that have the same encoding on big- and little-endian platforms.

The test case src/test/mir-opt/issues/issue_75439.rs is failing as well, but since the purpose of the test is to validate handling of big-endian integer encodings on a little-endian platform, it does not make much sense to run it on big-endian platforms in the first place - we can just ignore it there.

Fixed part of https://github.com/rust-lang/rust/issues/105383.