RISCVAsmParser: Don't treat operands with relocation specifier as parse-time constants

An immediate operand is encoded as an `MCExpr`, with `RISCVMCExpr`
specifying an operand that includes a relocation specifier. When
https://reviews.llvm.org/D23568 added initial fixup and relocation
support in 2017, it adapted code from `PPCMCExpr` (for `@l` `@ha`) to
evaluate the `RISCVMCExpr` operand. (PPCAsmParser had considerable
technical debt, though I’ve recently streamlined it somewhat, e.g.
8560da28c69de481f3ad147722577e87b902facb)

Evaluating RISCVMCExpr during parsing is unnecessary. For example,
there's no need to treat `lui a0, %hi(2)` differently from `lui a0,
%hi(foo)` when foo has not been defined yet.

This evaluation introduces unnecessary complexity. For instance, parser
functions require an extra check like `VK == RISCVMCExpr::VK_None`, as
seen in these examples:

```
 if (!evaluateConstantImm(getImm(), Imm, VK) || VK != RISCVMCExpr::VK_None)

 return IsConstantImm && isUInt<N>(Imm) && VK == RISCVMCExpr::VK_None;
```

This PR eliminates the parse-time evaluation of `RISCVMCExpr`, aligning
it more closely with other targets.

---

`abs = 0x12345; lui t3, %hi(abs)` now generates
R_RISCV_HI20/R_RISCV_RELAX with linker relaxation.
(Tested by test/MC/RISCV/linker-relaxation.s)

(Notably, since commit ba2de8f22d0cac86d89c1806fb54ed3463349342 in
lld/ELF, the linker can handle R_RISCV_HI relocations with a symbol
index of 0 in -pie mode.)

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

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