Improve naming of llvm Instructions created.

We now try harder to keep the names of instructions related to the
initial names of variables they're derived from and so forth.  This
is useful for making both LLVM IR as well as generated C++ code
easier to correlate back to the original ispc source code.

Issue #244.

cbackend.cpp

@@ -4402,7 +4402,8 @@ SmearCleanupPass::runOnBasicBlock(llvm::BasicBlock &bb) {
             Value *args[1] = { toMatch };
             ArrayRef<llvm::Value *> argArray(&args[0], &args[1]);
             Instruction *smearCall = 
-                CallInst::Create(smearFunc, argArray, "smear", (Instruction *)NULL);
+                CallInst::Create(smearFunc, argArray, LLVMGetName(toMatch, "_smear"),
+                                 (Instruction *)NULL);
 
             ReplaceInstWithInst(iter, smearCall);