LLVM generate llvm.expect for conditional branches

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