Fixes for the IR interpreter:

 - Implemented the SExt instruction, and

 - eliminated redundant codepaths for constant
   handling.

Added test cases.

<rdar://problem/13244258>
<rdar://problem/13955820>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@183344 91177308-0d34-0410-b5e6-96231b3b80d8