MC: Refine ALIGN relocation conditions

Each section now tracks the index of the first linker-relaxable
fragment, enabling two changes:

* Delete redundant ALIGN relocations before the first linker-relaxable
  instruction in a section. The primary example is the offset 0
  R_RISCV_ALIGN relocation for a text section aligned by 4.
* For alignments larger than the NOP size after the first
  linker-relaxable instruction, ALIGN relocations are now generated, even in
  norelax regions. This fixes the issue #150159.

The new test llvm/test/MC/RISCV/Relocations/align-after-relax.s
verifies the required ALIGN in a norelax region following
linker-relaxable instructions.
By using a fragment index within the subsection (which is less than or
equal to the section's index), the implementation may generate redundant
ALIGN relocations in lower-numbered subsections before the first
linker-relaxable instruction.

align-option-relax.s demonstrates the ALIGN optimization.
Add an initial `call` to a few tests to prevent the ALIGN optimization.

---

When the alignment exceeds 2, we insert $alignment-2 bytes of NOPs, even
in non-RVC code. This enables non-RVC code following RVC code to handle
a 2-byte adjustment without requiring an additional state in MCSection
or AsmParser.

```
.globl _start
_start:
// GNU ld can relax this to  6505          lui     a0, 0x1
// LLD hasn't implemented this transformation.
  lui a0, %hi(foo)

.option push
.option norelax
.option norvc
// Now we generate R_RISCV_ALIGN with addend 2, even if this is a norvc region.
.balign 4
b0:
  .word 0x3a393837
.option pop
foo:
```

Pull Request: https://github.com/llvm/llvm-project/pull/150816

2 files changed, 2 insertions, 2 deletions

diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 8c27958..d0c6144 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -443,7 +443,7 @@ void MCObjectStreamer::emitInstToData(const MCInst &Inst,
     // MCAssembler::relaxAlign.
     auto *Sec = F->getParent();
     if (!Sec->isLinkerRelaxable())
-      Sec->setLinkerRelaxable();
+      Sec->setFirstLinkerRelaxable(F->getLayoutOrder());
     // Do not add data after a linker-relaxable instruction. The difference
     // between a new label and a label at or before the linker-relaxable
     // instruction cannot be resolved at assemble-time.
diff --git a/llvm/lib/MC/MCSection.cpp b/llvm/lib/MC/MCSection.cpp
index 27ca131..9ed6fd1 100644
--- a/llvm/lib/MC/MCSection.cpp
+++ b/llvm/lib/MC/MCSection.cpp
@@ -20,7 +20,7 @@ using namespace llvm;
 
 MCSection::MCSection(StringRef Name, bool IsText, bool IsBss, MCSymbol *Begin)
     : Begin(Begin), HasInstructions(false), IsRegistered(false), IsText(IsText),
-      IsBss(IsBss), LinkerRelaxable(false), Name(Name) {
+      IsBss(IsBss), Name(Name) {
   DummyFragment.setParent(this);
 }