[Debug][AArch64] Do not crash on unknown subreg register sizes. (llvm#160442)

The AArch64 zsub regs are scalable, so defined with a size of -1 (which
comes through as 65535). The RegisterSize is only 128, so code to try
and find overlapping regs of a z30_z31 in DwarfEmitter can crash on
trying to access out of range bits in a BitVector. Hexagon and x86 also
contain subregs with unknown sizes.

Ideally most of these would be scalable values but in the meantime add a
check that the register are small enough to overlap with the current
register size, to prevent us from crashing.

This fixes the issue reported on llvm#153810.

Author: davemgreen

llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp

@@ -154,7 +154,7 @@ bool DwarfExpression::addMachineReg(const TargetRegisterInfo &TRI,
     unsigned Size = TRI.getSubRegIdxSize(Idx);
     unsigned Offset = TRI.getSubRegIdxOffset(Idx);
     Reg = TRI.getDwarfRegNum(SR, false);
-    if (Reg < 0)
+    if (Reg < 0 || Offset + Size > RegSize)
       continue;
 
     // Used to build the intersection between the bits we already