Don't treat a partial <def,undef> operand as a read.

Normally, a partial register def is treated as reading the
super-register unless it also defines the full register like this:

  %vreg110:sub_32bit<def> = COPY %vreg77:sub_32bit, %vreg110<imp-def>

This patch also uses the <undef> flag on partial defs to recognize
non-reading operands:

  %vreg110:sub_32bit<def,undef> = COPY %vreg77:sub_32bit

This fixes a subtle bug in RegisterCoalescer where LIS->shrinkToUses
would treat a coalesced copy as still reading the register, extending
the live range artificially.

My test case only works when I disable DCE so a dead copy is left for
RegisterCoalescer, so I am not including it.

<rdar://problem/9967101>

llvm-svn: 138018

lib/CodeGen/MachineInstr.cpp

@@ -901,7 +901,8 @@ MachineInstr::readsWritesVirtualRegister(unsigned Reg,
       Ops->push_back(i);
     if (MO.isUse())
       Use |= !MO.isUndef();
-    else if (MO.getSubReg())
+    else if (MO.getSubReg() && !MO.isUndef())
+      // A partial <def,undef> doesn't count as reading the register.
       PartDef = true;
     else
       FullDef = true;