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, 3 insertions, 1 deletions

diff --git a/llvm/lib/MC/MCELFStreamer.cpp b/llvm/lib/MC/MCELFStreamer.cpp
index 29499e7..17c8266 100644
--- a/llvm/lib/MC/MCELFStreamer.cpp
+++ b/llvm/lib/MC/MCELFStreamer.cpp
@@ -450,8 +450,10 @@ void MCELFStreamer::emitInstToData(const MCInst &Inst,
   auto Fixups = MutableArrayRef(DF->getFixups()).slice(FixupStartIndex);
   for (auto &Fixup : Fixups) {
     Fixup.setOffset(Fixup.getOffset() + CodeOffset);
-    if (Fixup.needsRelax())
+    if (Fixup.isLinkerRelaxable()) {
       DF->setLinkerRelaxable();
+      getCurrentSectionOnly()->setLinkerRelaxable();
+    }
   }
 
   DF->setHasInstructions(STI);