William Fisher:  A bug in __lookahead::exec causes /(?=^)b/ to match ab. When  makes a recursive call to , it passes true for the value of . This causes a beginning-of-line anchor (^) inside a lookahead assertion to match anywhere in the text.  This fixes http://llvm.org/bugs/show_bug.cgi?id=11118
Add flag to lli to enable debugging of IR when used with MCJIT.
- warn users when -debug-ir is used with old JIT engine (only partial debug
  info is available)

For example, to debug an IR file with GDB (that supports JIT registration), do:

$ gdb --args lli -use-mcjit -debug-ir testcase.ll
(gdb) break main
(gdb) run
<Process continues to lli main>
(gdb) continue
<Process continues to testcase.ll main()
(gdb) step
<Now stepping through the LLVM IR in testcase.ll>