[NFC] G_DYN_STACKALLOC realign iff align > 1, update documentation

[gchatelet]

Summary

I think it would be better to require the alignment to be >= 1. It is currently confusing to allow both values.

llvm/docs/GlobalISel/GenericOpcode.rst

.. caution::
   I found no documentation for this instruction at the time of writing.
 
 Other Operations
 ----------------
 
 G_DYN_STACKALLOC
 ^^^^^^^^^^^^^^^^
 
-Dynamically realign the stack pointer to the specified alignment
+Dynamically realigns the stack pointer to the specified size and alignment.
+An alignment value of `0` or `1` mean no specific alignment.
 
 .. code-block:: none
 
   %8:_(p0) = G_DYN_STACKALLOC %7(s64), 32
-
-.. caution::
-
-  What does it mean for the immediate to be 0? It happens in the tests

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

LegalizerHelper::lowerShuffleVector(MachineInstr &MI) {
   MI.eraseFromParent();
   return Legalized;
 }
 
 LegalizerHelper::LegalizeResult
 LegalizerHelper::lowerDynStackAlloc(MachineInstr &MI) {
   Register Dst = MI.getOperand(0).getReg();
   Register AllocSize = MI.getOperand(1).getReg();
-  unsigned Align = MI.getOperand(2).getImm();
+  Align Alignment = assumeAligned(MI.getOperand(2).getImm());
 
   const auto &MF = *MI.getMF();
   const auto &TLI = *MF.getSubtarget().getTargetLowering();
 
   LLT PtrTy = MRI.getType(Dst);
   LLT IntPtrTy = LLT::scalar(PtrTy.getSizeInBits());
 
   Register SPReg = TLI.getStackPointerRegisterToSaveRestore();
   auto SPTmp = MIRBuilder.buildCopy(PtrTy, SPReg);
   SPTmp = MIRBuilder.buildCast(IntPtrTy, SPTmp);
 
   // Subtract the final alloc from the SP. We use G_PTRTOINT here so we don't
   // have to generate an extra instruction to negate the alloc and then use
   // G_PTR_ADD to add the negative offset.
   auto Alloc = MIRBuilder.buildSub(IntPtrTy, SPTmp, AllocSize);
-  if (Align) {
-    APInt AlignMask(IntPtrTy.getSizeInBits(), Align, true);
+  if (Alignment > Align(1)) {
+    APInt AlignMask(IntPtrTy.getSizeInBits(), Alignment.value(), true);
     AlignMask.negate();
     auto AlignCst = MIRBuilder.buildConstant(IntPtrTy, AlignMask);
     Alloc = MIRBuilder.buildAnd(IntPtrTy, Alloc, AlignCst);
   }
 
   SPTmp = MIRBuilder.buildCast(PtrTy, Alloc);
   MIRBuilder.buildCopy(SPReg, SPTmp);
   MIRBuilder.buildCopy(Dst, SPTmp);