[llvm] RISCVAsmParser: Don't treat operands with relocation specifier as parse-time constants (PR #133377)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> I'm happy with the code but LLD seems pretty unhappy. Is the problem in the LLD tests one that GNU LD will also have (doesn't like HI/LO relocations against local symbols)

Looks like LLD should not give an error for the test ELF/riscv-relax-hi20-lo12-pie.s . I'll fix it separately.

In addition, this exposes some assembler differences:

```asm
// R_RISCV_HI20 after this patch
abs = 0x100000
lui x0, %hi(abs)

// R_RISCV_HI20, w/o or w/ this patch; GNU assembler doesn't emit a relocation
lui x0, %hi(abs)
abs = 0x100000

// gas suppresses the relocation even if abs is weak.
```

PowerPC suppresses R_PPC64_LO16 relocation at MCAssembler::evaluateFixup time. RISC-V can't due to relaxation. I prefer keeping things simple and avoiding the addition of assembler code to suppress the relocation.

https://github.com/llvm/llvm-project/pull/133377