This patch adds likeliness annotations to heap and and stack checks and
modifies the llvm codegen to recognize those to help it generate better
code.

So with this patch

```
...
if ((Sp + 8) - 24 < SpLim) (likely: False) goto c23c; else goto c23d;
...
```

roughly generates:

```
  %ln23k = icmp ult i64 %ln23j, %SpLim_Arg
  %ln23m = call ccc i1 (i1, i1) @llvm.expect.i1( i1 %ln23k, i1 0 )
  br i1 %ln23m, label %c23c, label %c23d
```

Note the call to `llvm.expect` which denotes the expected result for
the comparison.

Test Plan: Look at assembler code with and without this patch. If the
heap-checks moved out of the way we are happy.

Reviewers: austin, simonmar, bgamari

Reviewed By: bgamari

Subscribers: michalt, thomie

Differential Revision: https://phabricator.haskell.org/D2688

GHC Trac Issues: #8321