[NFC][llvm] Inclusive language: remove instance of master from Thumb2SizeReduction.cpp

[quinnp]

Summary

[NFC] As part of using inclusive language within the llvm project, this patch
replaces master with main in Thumb2SizeReduction.cpp.

[dmgreen]

Sure. Sounds good to me.

llvm/lib/Target/ARM/Thumb2SizeReduction.cpp

case ARM::t2MOVi16:
     // Can convert only 'pure' immediate operands, not immediates obtained as
     // globals' addresses.
     if (MI->getOperand(1).isImm())
       return ReduceToNarrow(MBB, MI, Entry, LiveCPSR, IsSelfLoop);
     break;
   case ARM::t2CMPrr: {
     // Try to reduce to the lo-reg only version first. Why there are two
     // versions of the instruction is a mystery.
-    // It would be nice to just have two entries in the master table that
+    // It would be nice to just have two entries in the main table that
     // are prioritized, but the table assumes a unique entry for each
     // source insn opcode. So for now, we hack a local entry record to use.
     static const ReduceEntry NarrowEntry =
       { ARM::t2CMPrr,ARM::tCMPr, 0, 0, 0, 1, 1,2, 0, 0,1,0 };
     if (ReduceToNarrow(MBB, MI, NarrowEntry, LiveCPSR, IsSelfLoop))
       return true;
     return ReduceToNarrow(MBB, MI, Entry, LiveCPSR, IsSelfLoop);
   }