InstrEmitter::EmitSubregNode() optimize extract_subreg in this case:

r1025 = s/zext r1024, 4
r1026 = extract_subreg r1025, 4

to a copy:
r1026 = copy r1024

This is correct. However it uses TII->isCoalescableExtInstr() which can return
true for instructions which essentially does a sext_in_reg so this can end up
with an illegal copy where the source and destination register classes do not
match. Add a check to avoid it. Sorry, no test case possible at this time.

rdar://11849816

llvm-svn: 160059

llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp

@@ -479,7 +479,8 @@ void InstrEmitter::EmitSubregNode(SDNode *Node,
     unsigned SrcReg, DstReg, DefSubIdx;
     if (DefMI &&
         TII->isCoalescableExtInstr(*DefMI, SrcReg, DstReg, DefSubIdx) &&
-        SubIdx == DefSubIdx) {
+        SubIdx == DefSubIdx &&
+        TRC == MRI->getRegClass(SrcReg)) {
       // Optimize these:
       // r1025 = s/zext r1024, 4
       // r1026 = extract_subreg r1025, 4