Fix RISC-V Test Failures in ./x test for Multiple Codegen Cases

[CaiWeiran]

This PR addresses and resolves several test failures that occurred when running ./x test on the RISC-V architecture. The issues were due to platform-specific behavior, ABI differences, or code generation inconsistencies specific to RISC-V.

The following tests have been fixed:

• codegen-llvm/enum/enum-match.rs
• codegen-llvm/enum/enum-transparent-extract.rs
• codegen-llvm/repeat-operand-zero-len.rs
• codegen-llvm/enum/enum-aggregate.rs
• codegen-llvm/uninhabited-transparent-return-abi.rs

All changes have been tested locally with ./x test on a RISC-V target and now pass as expected.

Notes:

• These fixes are scoped specifically to enable full test suite compliance for RISC-V targets.
• No changes were made that affect other architectures.

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[compiler-errors]

Please squash this, and it would be useful if you actually described what effect these changes had and why they're only important for RISCV codegen.

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[rust-bors]

@CaiWeiran: 🔑 Insufficient privileges: not in try users

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[bors]

☔ The latest upstream changes (presumably #145423) made this pull request unmergeable. Please resolve the merge conflicts.