RISCV: Remove shouldForceRelocation and unneeded relocations

Follow-up to #140494

`shouldForceRelocation` is conservative and produces redundant
relocations.

For example, RISCVAsmBackend::ForceRelocs (introduced to support mixed
relax/norelax code) leads to redundant relocations in the following
example adapted from #77436

```
.option norelax
j label
// For assembly input, RISCVAsmParser::ParseInstruction sets ForceRelocs (https://reviews.llvm.org/D46423).
// For direct object emission, RISCVELFStreamer sets ForceRelocs (#77436)
.option relax
call foo  // linker-relaxable

.option norelax
j label   // redundant relocation due to ForceRelocs
.option relax

label:
```

Root problem: The `isSymbolRefDifferenceFullyResolvedImpl` condition in
MCAssembler::evaluateFixup does not check whether two locations are
separated by a fragment whose size can be indeterminate due to linker
instruction (e.g. MCDataFragment with relaxation, or MCAlignFragment
due to indeterminate start offst).

This patch

* Updates the fragment walk code in
  `attemptToFoldSymbolOffsetDifference` to treat MCRelaxableFragment
  (for --riscv-asm-relax-branches) as fixed size after finishLayout.
* Adds a condition in `addReloc` to complement
  `isSymbolRefDifferenceFullyResolvedImpl`.
* Removes the no longer needed `shouldForceRelocation`.

This fragment walk code path handles nicely handles
mixed relax/norelax case from
https://discourse.llvm.org/t/possible-problem-related-to-subtarget-usage/75283
and allows us to remove `MCSubtargetInfo` argument (#73721) as a follow-up.

This fragment walk code should be avoided in the absence of
linker-relaxable fragments within the current section.

Adjust two bolt/test/RISCV tests (#141310)

Pull Request: https://github.com/llvm/llvm-project/pull/140692

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
index 4c159fe..765f1a2 100644
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -390,6 +390,12 @@ static void attemptToFoldSymbolOffsetDifference(const MCAssembler *Asm,
       unsigned Count;
       if (DF) {
         Displacement += DF->getContents().size();
+      } else if (auto *RF = dyn_cast<MCRelaxableFragment>(FI);
+                 RF && Asm->hasFinalLayout()) {
+        // Before finishLayout, a relaxable fragment's size is indeterminate.
+        // After layout, during relocation generation, it can be treated as a
+        // data fragment.
+        Displacement += RF->getContents().size();
       } else if (auto *AF = dyn_cast<MCAlignFragment>(FI);
                  AF && Layout && AF->hasEmitNops() &&
                  !Asm->getBackend().shouldInsertExtraNopBytesForCodeAlign(