[RISCV] Considering existing offset in the alignment when folding ADDIs into load/store.

[craig.topper]

Summary

getPointerAlignment and ConstantPoolSDNode::getAlign only consider
the alignment of the object. If we already have a non-zero offset
into the offset that may have reduced the alignment.

Since the base pointer will become an LUI with the old offset, we
need to be sure the new offset fits in the alignment of the address
that will be used to create the LUI immediate.

I'm not sure it is possible to have a non-zero offset in the
GlobalAddressSDNode or ConstantPoolSDNode at this point today so this
may only be a theoretical bug.

[reames]

LGTM

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

if (auto *Const = dyn_cast<ConstantSDNode>(ImmOperand)) {
     ImmOperand = CurDAG->getTargetConstant(CombinedOffset, SDLoc(ImmOperand),
                                            ImmOperand.getValueType());
   } else if (auto *GA = dyn_cast<GlobalAddressSDNode>(ImmOperand)) {
     // If the off1 in (addi base, off1) is a global variable's address (its
     // low part, really), then we can rely on the alignment of that variable
     // to provide a margin of safety before off1 can overflow the 12 bits.
     // Check if off2 falls within that margin; if so off1+off2 can't overflow.
     const DataLayout &DL = CurDAG->getDataLayout();
-    Align Alignment = GA->getGlobal()->getPointerAlignment(DL);
+    Align Alignment = commonAlignment(GA->getGlobal()->getPointerAlignment(DL),
+                                      GA->getOffset());
     if (Offset2 != 0 && Alignment <= Offset2)
       return false;
     int64_t Offset1 = GA->getOffset();
     int64_t CombinedOffset = Offset1 + Offset2;
     ImmOperand = CurDAG->getTargetGlobalAddress(
         GA->getGlobal(), SDLoc(ImmOperand), ImmOperand.getValueType(),
         CombinedOffset, GA->getTargetFlags());
   } else if (auto *CP = dyn_cast<ConstantPoolSDNode>(ImmOperand)) {
     // Ditto.
-    Align Alignment = CP->getAlign();
+    Align Alignment = commonAlignment(CP->getAlign(), CP->getOffset());
     if (Offset2 != 0 && Alignment <= Offset2)
       return false;
     int64_t Offset1 = CP->getOffset();
     int64_t CombinedOffset = Offset1 + Offset2;
     ImmOperand = CurDAG->getTargetConstantPool(
         CP->getConstVal(), ImmOperand.getValueType(), CP->getAlign(),
         CombinedOffset, CP->getTargetFlags());
   } else {