Fix compilation against LLVM 3.4

On Mint Linux x86_64, compilation was failing. (LLVM 3.4, and Clang 3.4, were used.)

This commit fixes the build. Regarding whether things actually work: most the regressions tests seem to succeed, but:

test_structs_as_args produces:
```c++
ERROR: clEnqueueNDRangeKernel(-52)
```

test_constant_array produces:
```c++
/tmp/poclnrWIKE/program.cl:7:10: error: passing 'char *' to parameter of type '__constant char *' changes address space of pointer
  printf("sum=%g\n", s);
         ^~~~~~~~~~
/usr/local/share/pocl/include/_kernel.h:2123:40: note: passing argument to parameter 'format' here
int _cl_printf(constant char* restrict format, ...);
                                       ^
1 error generated.
ERROR: clBuildProgram(-11)
```


test_infinite_loop produces:
```c++
/tmp/poclAjAOiO/program.cl:7:10: error: passing 'char *' to parameter of type '__constant char *' changes address space of pointer
  printf("sum=%g\n", s);
         ^~~~~~~~~~
/usr/local/share/pocl/include/_kernel.h:2123:40: note: passing argument to parameter 'format' here
int _cl_printf(constant char* restrict format, ...);
                                       ^
1 error generated.
ERROR: clBuildProgram(-11)
```

Still more tests fail outside the regression-tests collection.

Without my changes to TargetAddressSpaces.cc, I see the following (from g++ version 4.8.1):

```c++
 TargetAddressSpaces.cc: In member function 'virtual bool pocl::TargetAddressSpaces::runOnModule(llvm::Module&)':
 TargetAddressSpaces.cc:202:17: error: 'AddrSpaceCastInst' was not declared in this scope
          if (isa<AddrSpaceCastInst>(instr)) {
                  ^
 TargetAddressSpaces.cc:202:41: error: no matching function for call to 'isa(llvm::Instruction*&)'
          if (isa<AddrSpaceCastInst>(instr)) {
                                          ^
 TargetAddressSpaces.cc:202:41: note: candidate is:
 In file included from /usr/lib/llvm-3.4/include/llvm/IR/Type.h:19:0,
                  from /usr/lib/llvm-3.4/include/llvm/IR/DerivedTypes.h:21,
                  from /usr/lib/llvm-3.4/include/llvm/IR/Instructions.h:23,
                  from TargetAddressSpaces.cc:31:
 /usr/lib/llvm-3.4/include/llvm/Support/Casting.h:134:13: note: template<class X, class Y> bool llvm::isa(const Y&)
  inline bool isa(const Y &Val) {
              ^
 /usr/lib/llvm-3.4/include/llvm/Support/Casting.h:134:13: note:   template argument deduction/substitution failed:
 TargetAddressSpaces.cc:202:41: error: template argument 1 is invalid
          if (isa<AddrSpaceCastInst>(instr)) {
                                          ^
```

Are you sure you are not having pre 3.4 release LLVM or somehow mixed installation of pre 3.4 LLVM (e.g. from Ubuntu packages)? Otherwise, it seems strange it does not build for you as LLVM 3.4 is the main tested version with pocl now and I for one use it all the time with pocl without troubles.