Hexagon: Remove implicit ilist iterator conversions, NFC

There are two things out of the ordinary in this commit.  First, I made
a loop obviously "infinite" in HexagonInstrInfo.cpp.  After checking if
an instruction was at the beginning of a basic block (in which case,
`break`), the loop decremented and checked the iterator for `nullptr` as
the loop condition.  This has never been possible (the prev pointers are
always been circular, so even with the weird ilist/iplist
implementation, this isn't been possible), so I removed the condition.

Second, in HexagonAsmPrinter.cpp there was another case of comparing a
`MachineBasicBlock::instr_iterator` against `MachineBasicBlock::end()`
(which returns `MachineBasicBlock::iterator`).  While not incorrect,
it's fragile.  I switched this to `::instr_end()`.

All that said, no functionality change intended here.

llvm-svn: 250778
diff --git a/llvm/lib/Target/Hexagon/HexagonSplitConst32AndConst64.cpp b/llvm/lib/Target/Hexagon/HexagonSplitConst32AndConst64.cpp
index d3eb56f..10fe606 100644
--- a/llvm/lib/Target/Hexagon/HexagonSplitConst32AndConst64.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonSplitConst32AndConst64.cpp
@@ -81,7 +81,7 @@
   // Loop over all of the basic blocks
   for (MachineFunction::iterator MBBb = Fn.begin(), MBBe = Fn.end();
        MBBb != MBBe; ++MBBb) {
-    MachineBasicBlock* MBB = MBBb;
+    MachineBasicBlock *MBB = &*MBBb;
     // Traverse the basic block
     MachineBasicBlock::iterator MII = MBB->begin();
     MachineBasicBlock::iterator MIE = MBB->end ();