* How to add an llvm intrinsic

  - add enum value llvmintrinsic::foo to llvmintrinsic in xo-expression
  - also add foo to llvmintrinsic2str in xo-expression llvmintrinsic.hpp
  - if we have a built-in Primitive for the same functionality,
    want Primitive::intrinsic_ = llvmintrinsic::foo
  - in MachPipeline::codegen_apply(), look for switch(intrinsic),
    add case llvmintrinsic::foo
  - substitute codegen for the intrinsic
    in place of the catch-all IRBuilder::CreateCall

** To test from python:

   1. install xo-pyjit and deps somewhere (~/local2 in this example)

   2. PYTHONPATH=~/local2:$PYTHONPATH python

   3. python:

      from xo_pyreflect import *
      from xo_pyexpression import *
      from xo_pyjit import *
      i32_t=TypeDescr.lookup_by_name('double')
      x=make_var('x',i32_t)
      f=make_mul_i32_pm()
      c=make_apply(f,[x,x])
      lm=make_lambda('sq',[x],c)

      mp=MachPipeline.make()
      code=mp.codegen(lm)
      print(code.print())

   4. in our example,  get output like:

      define i32 @sq(i32 %x) {
      entry:
        %0 = mul i32 %x, %x
        ret i32 %0
      }

   5. to compile+run via JIT:

      mp.machgen_current_module()
      fn=mp.lookup_fn('int (*)(int)', 'sq')
      fn(16) # -> 256

** to figure out what 'IR should look like' for something simple

   write some c++:

    #include <cmath>

    struct env_type {
        env_type * parent;
        env_type * (*unwind)(env_type *, int);
    };

    double wrap_sqrt(env_type * env, double x) {
        return ::sqrt(x);
    }

    int main() {
        wrap_sqrt(nullptr, 2.0);
    }

   compile to emit IR

    $ clang -cc1 ex_cpp.cpp -emit-llvm

   inspect

    ex_cpp.ll